Green’s Dictionary of Slang

(US, Western) to shoot someone and roll their body in the river.

1929		J.F. Dobie A Vaquero of the Brush Country 292: He ran the risk of being ‘pecosed’ either for his integrity or lack of it.
1936		R.F. Adams Cowboy Lingo 171: To ‘pecos’ one meant to kill him and roll his body into the river.
1944		R.F. Adams Western Words (1968) 113: Pecos. [...] To shoot a man and roll his body into the river.