Green’s Dictionary of Slang

(US) to knock out; thus n. used of the victim.

[1719		Proc. Old Bailey 25 Feb. n.p.: One of the Prisoners chuck’d the Deceased under the Chin, saying he would cock his Chin].
1895		Crowe Aus. Sl. Dict. 18: Cock, a term in the prize-ring for a man knocked out of time.
1930		W.R. Burnett Iron Man 12: I cocked him through the ropes in the second.