“Pecosin’” is an old-time term that means to kill someone and throw the body into the Pecos River, often with weights so it drowns. Later, the term “Pecosin’” (or the verb “Pecos”) came to be used for dumping a body in any river. The term can also means to douse a sleeping person with water.
 
A “Pecos swap” was a euphemism for stealing. Both terms “Pecos” and “Pecos swap” are of historical interest today.
 
 
JSTOR
“The Lighter Side of Texas Place Naming”
Harold Clay Pope
Western Folklore, Vol. 13, No. 2/3 (1954), pp. 125-129
“Pecosin’ a feller” meant killing a person, weighting the body with rocks, and dropping it in the Pecos River.
 
University of Texas of the Permian Basin; Roadlogs
Pecos city limit and overpass.  Pecos was established in 1881 as a stop on the Texas and Pacific Railroad.  It was first called Pecos Station, then Pecos City, and finally Pecos.  Prior to county organization their was a period of lawlessness; the expression “Pecosin” came to mean killing a man and throwing his body in the Pecos River.
     
A Vaquero of the Brush County
by J. Frank Dobie
Dallas, TX: The Southwest Press
1929
Pg. 277:
[ref. to 1880s—ed.] “He ran the risk of being “pecosed” either for his integrity of the lack of it….To “pecos” a man one shot him and rolled his body into the river.”
 
El Diablo Cojo (The Limping Devil)
by Patrick Sylvester McGeeney
San Antonio, TX: Standard Printing Company
1936
Pg. 39:
“Fetch a bucket of water an’ we’ll Pecos him, wake him up, an’ then we’ll find out what stampeded him….Slats doused Ebony Jo with an overabundance of water.”
 
28 May 1936, El Paso (TX) Herald-Post, pg. 2D, col. 5:
It was the men of Miller’s stamp that led to the coining of the expression, “Pecosing a fellow,” which meant to murder the man in question, load his body with rocks, and drop it into the rushing waters of the river which knew how to keep secrets.
 
21 May 1962, Dallas Morning News, “Tolbert’s Texas” by Frank X. Tolbert, section 4, pg. 1:
WONDER WHY THE ancient and sinister term, “pecosin” hasn’t crept into some of the millions of words about Pecos’ most-written-about inhabitant, Billy Sol Estes? In the Old West, “pecosin” came to mean killing a man and throwing his body into the handiest river. In its original form it meant tossing the victim into the Pecos River, of course.
   
28 May 1962, Dallas Morning News, “Tolbert’s Texas” by Frank X. Tolbert, section 4, pg. 1:
Mr. Kerr Doubts
Term, “Pecosin’”
By Frank X. Tolbert
 
WALTER L. KERR, a Midland lawyer, has written this department a letter, suggesting that I just rared back and made up the term, “Pecosin’,” meaning to shoot a man and get shed of the evidence by throwing his body in a river. (I was wondering why “Pecosin’” hadn’t cropped up in all the millions of words of prose composed about the adventures of Pecos’ Billy Sol Estes.)
 
“I was raise in Pecos and I never heard of Pecosin’,” declared Mr. Kerr in his letter.
 
Well, Mr. Kerr, this word is sprinkled through the literature on the Old West. I can give you two short-order references on it. On Page 113 of his authoritative book, “Western Words, a Dictionary of the Range, Cow Camp, and Trail”, Ramon F. Adams wrote that “to Pecos” a man meant “to shoot him and roll his body into the river.” Also, on Page 354, Volume 2, of “The Handbook of Texas”, edited by Dr. Walter Prescott Webb and other heavy hitters in the historical game, there is a short definition of the term. This is in an article on the town of Pecos, and it reads: “A period of lawlessness preceeded county organization; the expression ‘Pecosin’’ came to mean killing a man and throwing his body into the Pecos River.”
 
CALLED MR. ADAMS and discussed “Pecosin’.” “The word was common in the old days all along the river and not just in the town of Pecos,” he ssaid. “I once talked with a pioneer New Mexico cowboy, Jack Potter, who spoke often about unfortunate fellows who were ‘Pecosed’. It seems that the bodies were usually weighted with rocks.”
 
Mr. Adams mentioned another western phrase, “Pecos swap”, which might be handy for use in stories about Billie Sol. A “Pecos swap”, says Adams, meant “a trade made without consent or knowledge of the other intended party.” It meant “to steal”.
 
Also, Pecos is a Spanish-ized Indian word meaning “crooked”. The Pecos was sometimes called “Dirty River” in the olden days.
 
COUNSELLOR KERR need not feel too badly over his ignorance of “Pecosin’”. Allen Propp, general manager of the Pecos Independent, told me he’d lived all his life in the town of Pecos and had never heard the term. Guess Mr. Kerr and Mr. Propp weren’t much given to talk with old-timers.
 
12 June 1962, Dallas Morning News, “Tolbert’s Texas” by Frank X. Tolbert, section 4, pg. 14:
M. M. FULMER, pastor of the First Baptist Church in Goliad, comes to this department’s defense in the matter of the old term, “Pecosin’”, meaning to kil someone and throw the body in the river (the Pecos River in the original meaning o the word). A west Texas lawyer who had lived for a long time in Pecos said he’d never heard of “Pecosin’” and accused me of making it up.
 
“I served as pastor of the First Baptist Church in Ozona for seven years (1927-34) and was often in the company of older ranchmen,” said M/ M. Fulmer. “The term, ‘Pecosin’,’ was quite familiarly used at that time. I held a revival at Buena Vista in Pecos County. As a result, it was my pleasure to baptize 12 candidates in the Pecos River. I came in for quite a bit of teasing about ‘Pecosin’’ these people.”
   
Crossing Rio Pecos
by Patrick Dearen
Fort Worth, TX: Texas Christian University Press
1996
Pg. 18:
[ref. to 1872—ed.] Citizens and military authorities came to know them as perpetrators of a “Pecos swap.”…trading violence for four horses from cattle drove bound for El Paso.…A man named Little fell victim to pecosing…[He] reportedly guarded $24,000….A cowboy murdered him for the bonanza.