Green’s Dictionary of Slang

1. to rob in a reckless manner; thus cowboying adj.

1942		N. Algren Never Come Morning (1988) 18: Here he was, going somewhere but he didn’t know where, on some sort of cowboying job he didn’t know a thing about.
1954		(con. 1940) C. Chessman Cell 2455 175: So you cowboy it; you rob everything and anything in the way of business establishments.
1965		H. Williamson Hustler 196: ‘[R]ay feelin’’ [...] where you don’t plan what you’re goin’ a do, you just go out and catch whatever you can. You take more chances than you should, too. They also call this ‘cowboyin’,’ but ‘cowboyin’’ really means not taking as many chances.

2. to murder, to gun down.

1946		C.A. Wyer Sun (N.Y.) n.p.: He had had an assignment to kill a man and his bodyguard ‘even if we had to cowboy them.’ ‘What does that mean?’ asked Liebowitz. ‘That means that we were to kill them any place we found them even if it was in the middle of Broadway.’ [W&F].
1959		W. Burroughs Naked Lunch (1968) 233: Cowboy: New York hood talk means kill the mother fucker wherever you find him.
1977		E. Torres Q&A 180: ‘[T]he wops are gonna cowboy me on sight. Open contract’.

3. (also cowboy it) to act in a reckless manner; thus cowboying n.

1949–51		in M. Daly Profile of Youth 50: Despite the cowboying and hot rods, no proof can be obtained that high-schoolers are more prone to accidents than drivers in any other age range.
1998		(con. 1986) G. Pelecanos Sweet Forever 94: Tutt would cowboy it without thought if anything went down.