The “dead man’s hand” is a two-pair poker hand that also appears in Texas hold ‘em. The hand is widely believed to be aces and eights, but early citations show jacks and eights, tens and treys, tens and jacks, and other combinations.
 
It’s a modern myth that Wild Bill Hickok was playing poker when he was shot and that the term “dead man’s hand” derives from him. Hickok died in 1876, “dead man’s hand” is cited from 1886, and the association between “dead man’s hand” and Hickok is first cited from the 1920s (well after the fact).
 
   
Wikipedia: Dead Man’s Hand
In poker, the dead man’s hand is a two-pair hand, namely “aces and eights.” The origin of the name, legend has it, is the five-card-draw hand held by Wild Bill Hickok at the time of his murder, which is accepted to have included the aces and eights of both of the black suits (sometimes considered “bullets”), although his biographer, Rosa, says no contemporary cite for his hand has ever been found. The term “dead man’s hand” certainly predates the Hickok connection which only occurred in the mid-1920’s. Prior to this, the term referred to a variety of hands. The earliest found reference to a “dead man’s hand” is 1886, where it was described as “three jacks and a pair of tens.”

There are various claims as to the identity of Hickok’s fifth card, and there is also some reason to believe that he had discarded one card, the draw was interrupted by the shooting, and he never got the fifth card due to him.
 
Wikipedia: Wild Bill Hickok
James Butler Hickok (May 27, 1837 – August 2, 1876), better known as Wild Bill Hickok, was a legendary figure in the American Wild West. He is perhaps the best known figure from that era. After fighting in the Union army during the American Civil War, he became a legendary army scout, and later, lawman and gunfighter.

Hickok was born in Troy Grove, Illinois on May 27, 1837. He left his father’s farm in 1855 to be a stage coach driver on the Santa Fe and Oregon Trails. His gunfighting skills led to his nickname, “Wild Bill”.
(...)
Death
On August 2, 1876, while playing poker at Nuttal & Mann’s Saloon No. 10 in Deadwood (then part of the Dakota Territory but on Indian land), Hickok could not find an empty seat in the corner, where he always sat in order to protect himself against sneak attacks from behind, and instead sat with his back to the door. His paranoia was prescient: he was shot in the back of the head with a .45-caliber revolver by Jack McCall. Legend has it that Hickok, playing poker when he was shot, was holding a pair of aces and a pair of eights, with the fifth card disputed. The fifth card was either unknown, or some say that it had not yet been dealt. The game was interrupted by Hickok getting shot.
 
28 March 1886, Boston Daily Globe, “The Poker Craze” (from the Baltimore Herald), pg. 9:
I was present at a game in a Senator’s house one night and saw him win $6,000 on one hand. It was the dead man’s hand. What is the dead man’s hand? Why, it is three jacks and a pair of tens.  It is called the dead man’s hand because about forty seven years ago, in a town in Illinois, a celebrated judge bet his house and lot on three jacks and a pair of tens. (...) When his opponent showed up he had three queens and a pair of tens. Upon seeing the queens the judge fell back dead, clutching the jacks and tens in his hand, and that’s why a jack-full on tens is called the dead man’s hand.
 
13 April 1888, Atchison (KS) Daily Globe, pg. 1, col. 6:
“I held ‘aces and eights,’ a dead man’s hand,” was a note left by a Leavenworth suicide the other day.
 
13 July 1891, Woodland (CA) Daily Democrat, p. 4 (?), col. 4
[attrib. “New York Telegram”]
“I have drawn a pair of sevens. I now hold jacks full on red sevens. It is a fatal hand. No one ever yet held it and left the card table alive.”
 
28 July 1895, Atlanta Constitution, p. 23 (?), col. 3:
JACKS UP ON EIGHTS.
(...)
Finally Cherokee said: “I ain’t aimin’ to invest Wolfville in no superstitious fears, but I jest chronicles as a current event how I was settin’ into a little porker [sic] last night an’ three times straight I
picks up ‘the hand the dead man held’—jacks up on eights, an’ it win every time.”
 
13 August 1900, North Adams (MA) Transcript, pg. 6, col. 4:
TENS AND TREYS.
The Incident That Gave Them the
Name “Dead Man’s Hand.”
 
The term “Dead man’s hand” in poker means tens and treys, and the hand got its name because they won when the man that held them was dead.
 
Many years ago a party of planters and cowmen had a sitting on one of the boats plying up and down the Mississippi. Among them was a young fellow whose mother had sent him to New Orleans to bring back a large sum of money with which to pay off a mortgage on the homestead. He may be called Smith.
 
Bad luck and bad whisky very soon knocked a hole in his wad bigger than a pound of dry wool. Along in the night some one opened a fat, juicy pot for the “Downs,” and every one staid. It being Smith’s last say, he did the usual elevating, and every one staid again. This effort took about half of what was left of the original sum. We then drew cards. The opener drew three, the next two drew two cards each, and the last three drew one each. Smith was included in the one card draw. The opener then bet. Smith promptly raised with all the money he had left. As he was a rank bluffer the other players sized him up pretty close, and he was called by the opener, one of the two card men, and two of the one card men. He now stood to win about twice as much as he had originally. If he lost, he might as well shoot himself, for he could never face his mother without the money for the mortgage. After the last bet he laid his cards face down on the table in front of him and bent over the table with his head wresting on his arms.
 
The opener didn’t help his pair, another played made a pair of aces by drawing to a four flush, a third had two pair, sevens and fives, and the fourth had a pair of kings and an ace kicker. As Smith did not answer when asked to show his hand and as he was supposed to have fallen asleep a man named Halpin turned his cards over and showed a pair of tens an a pair of threes, which, being the big hand, entitled Smith to the pot. After vainly trying to rouse him one of the players, a doctor, raised his head and, looking long and carefully, pronounced him dead of heart disease. The money was sent to his mother, and Smith finished his trip as freight.
 
Since then tens and treys have been called the dead man’s hand, and it is a hand that is rarely beaten in draw poker. Aces and eights are called the gambler’s hand and is popularly supposed to be invincible.—New York Sun.
 
3 January 1903, National Police Gazette, pg. 6:
DEATH IN A POKER GAME WHERE ONE
PLAYER HELD A “DEAD MAN’S HAND.” 
A Jack Full on Red Sevens Seems to be an Unusually Fatal Combination of the Pasteboards. ... Half a dozen sports were lined up against the bar of a big hotel in Milwaukee, Wis., the other
night, and the talk drifted to poker. “There is in the great game,” remarked one, “what is known as the ‘Dead Man’s Hand.’ I only saw it played once, but that was enough for me.  I’d heard about this ‘Dead Man’s Hand,’ soon after I struck the West.
 
 
Google Books
Encyclopaedia of Superstitions, Folklore, and the Occult Sciences of the World
Milwaukee, WI: J. H. Yewdale & Sons
1903
Pg. 1478:
Jacks and sevens are called the “dead man’s hand.”  In a poker game, it is very unlucky to hold them and win the pot.
 
3 November 1904, Lincoln (NE) Evening News, pg.8, col. 5:
AN OUTCAST STEER WITH
ITS TRAGIC MARKINGS
ALPINE, Tex., Nov. 3.
(...)
In January, 1889. a big roundup took place on the Leon Cipa ranch, in Brewster County.
(...)
Some time before Gilliliand’s death, “Jeff” Webb, a nephew of Gillilian, left Alpine with a pet bear. The morning following his dead body was found on the road towards Fort Davis. Gilliliand has suspected that Webb had been killed by Sam Taylor, a desperado. One night not long after this, Taylor was killed in the back room of a saloon at Alpine playing poker by someone who fired a load of buckshot through the window. He leaned forward over the table, the cards still grasped in his lifeless hand. He had just won a pot with aces and eights, which was thereafter known as a “dead man’s hand.”
 
3 December 1905, Daily Nevada State Journal (Reno, NV), pg. 12, col. 4:
THE man who held a pair of jacks and a pair of eights in a recent poker game at Pittsburg would doubtless give assent to the accuracy of the superstition that calls such a combination “the dead man’s hand,” only that he is unfortunately in the usual position of the dead man—unable to speak.
   
26 August 1906, Syracuse (NY) Herald, “The Dead Man’s Hand,” pg. 40:
In our story the dead man’s hand is jacks and eights. A great many people say that jacks fall of red sevens is the real dead man’s hand and those who have faith in their belief that this is the hand will tell you to this day and hour that the man who holds this hand in a game of poker will never get up from the table alive. Then others say it is aces and eights, while more insist that two jacks and two sevens is the dead man’s hand. We are not disputing any of them.
 
There is a tragedy connected with our tale. The writer’s informants insist that it is an actual and real happening and the story of the game has come to us from several other sources, some of which we have reasons for accepting as reliable. Accept its authenticity if you are in the humor; believe it if you will, what’s the diff?
 
I can not give you any day, date or place, but will leave these important features to the reader’s imagination. Neither can names be stated, but the game is supposed to have been played in a Western town and the time is some years back.
 
3 January 1909, Washington Post (from the New York Telegraph), pg. 31, col. 6:
Once when he made a bet and Mr. Finegelmann called with a dubious feeling, Mr. Saltpeter said: “I win; I got a straight and you got only the dead man’s hand.”
 
And when Mr. Finegelmann showed down his jacks and eights the others looked at Mr. Saltpeter with a little awe and regarded him as some one uncanny.
   
9 October 1910, Washington Post, “Played the Dead Man’s Hand” from the New York Telegraph, Miscellany section, pg. 1:
“The dead man’s hand,” said Mr. Saltpeter.
 
“Where you been living?” said Mr. Rosenthal. “Call yourself a Cochem and a sport, and not know a hunch on the dead man’s hand. Such an ignorance.”
 
“What is it?” asked Mr. Fleugelmann.
 
“Aces and eights,” said Mr. Saltpeter. “Some say it’s jacks and eights, awer I know better—it’s aces and eights.”
 
29 March 1942, Charleston (WV) Gazette, “The Brighter Side” by Damon Runyon, pg. 6:
In certain western cities the poker players used to accept both aces and eights and jacks and eights as the dead man’s hand but differentiated between the two by calling jacks and eights the Montana dead man’s hand. As we understood it, a man in Montana died holding jacks and eights and the westerners seemed to feel he was entitled to as much distinction as the guy who passed away behind the aces and eights. Personally, we are inclined to think that the euphonic quality of the latter combination made it famous rather than any mortuary circumstances surrounding the holding thereof.