“Snake poison” was an 1800s nickname for whiskey. It’s easy to see why. There was a rumor that the only antidote for a snake bite was whiskey, so all the cowboys stocked up.
 
Texas was well-known for its snakes, and what could be better medicine than whiskey? The “whiskey cure” was quickly de-bunked, but cowboys kept drinking “snake poison” all the same.
 
 
(Oxford English Dictionary)
snake poison U.S. and Austral. slang, whisky
1890 L. D’OYLE Notches 4 It was variously called for as tangle-foot, *snake-poison,..chain-lightning, or other fancy name, but it was never called for as whisky.
   
15 June 1861, Scientific American, “Antidote for Insect and Snake Bites, pg. 372:
On page 355, Vol. XII (old series), of the SCIENTIFIC AMERICAN, a correspondent residing at Centreville, Miss., states that the best remedy he ever saw tried for snake poison is whisky and red pepper. A table spoonful of the pepper is added to a pint of whisky. This quantity will generally cure a full grown man who has been bitten; two such doses, at the most, will certainly effect a cure. This is a common remedy in various parts of Mississippi.
 
27 February 1879, New-York Evangelist, “Winter in Texas,” pg. 1:
Almost every one has his little bottle of whiskey hid away in a corner, waiting for a snake-bite. ‘Tis strange, yet ‘tis true, that that which is so poisonous and deadly in itself is the safest antidote for snake poison. Many lives were saved here last snake season by that which is so often and so deservedly cried down in press and pulpit; and as a result, it has its warm friends among us, who speak of it “tenderly.”
   
July 1879, Southern Planter and Farmer, pg. 378:
Get some whiskey, brandy or wine; mix with water equal parts of each, add to each tumblerful a teaspoonful of ammonia; give the human patient a tumblerful at a time, every few minutes, or drench the horse with three or four pints every twenty or thirty minutes, else he will be dead in from thirty minutes to three or four hours.
 
10 November 1888, Scientific American, “Whisky not an Antidote to Rattlesnake Poison,” pg. 298:
Dr. A. T. Hudson, of Stockton, Cal., writes: “Having often seen the statement made in the public press, and sometimes in medical journals, that whisky and ammonia were the acknowledged antidotes to rattlesnake bite, I feel it a duty to administer a corrective to the above fallacious teaching. ...”
   
15 April 1909, Washington Post, pg. 6:
SCOFFS AT WHISKY CURE.
Philadelphia Professor Declares It Use-
less for Rattlesnake Bite.
Philadelphia Dispatch to the New York Press.
 
Dr. John Marshall, professor of chemistry and toxicology in the University of Pennsylvania, Tuesday scoffed at the use of whisky in cases of rattlesnake bite. he said that all the popular methods of treating for snake poison were ineefective, except in so influencing the patient’s mind as to save him from death by fright.
 
“Whisky, permanganate of potash, and caustics are the popular remedies for the poison of the rattlesnake,” said Dr. Marshall, “but as antidotes they are useless. It is true that permanganate of potash will decompose the venom in the wound, but it does not follow the poison into the circulation. If it did it would kill of itself, because it will decompose the blood as well as snake poison. A blood serum is the only remedy which promises life to the victim of a rattlesnake. This serum is obtained by inoculating a horse with snake poison. If a man takes the serum and a hypodermic syringe with him into the haunts of rattlesnakes he runs a good chance of coming out alive. But to dose with whisky, as is the invariable practice, is simply to leave the poison to run its course and gamble with death.”
 
3 October 1976, Chicago Tribune, “J. L. Dillard, colloquially speaking” by Ciana Milesko-Pytel (Book review of Dillard’s American Talk: Where Our Words Came From), pg. F5:
The Old West is a dramatic subject, an the author provides another look at our colorful past in his chapter on drinking, smoking, and chewing tobacco. He gives us the imaginative names used for liquor such as redeye, red whisky, tangle-foot whisky, snake poison, chain lightning and valley tan, as well as the source of “bootleg,” “Boot Hill,” and other Western colloquialisms.