Green’s Dictionary of Slang

cap n.6

[they ‘cap’ the body]
(US black)

1. the mind; implied in snap one’s cap

2. the top of the head, the cranium; implied at peel caps under peel v.

In phrases

snap one’s cap (v.) (also bust one’s cap, pop one’s cap)

1. to lose one’s temper; to break down emotionally.

[US]Pittsburgh Courier (PA) 27 Feb. 20/1: Roberta Felder [...] snapped her cap when the WAACs’ doctor told her TB had set in.
[US]Mezzrow & Wolfe Really the Blues 187: You snapped your cap long ago.
[US]M. Clifton They’d Rather Be Right 148: The kid soldier had apparently snapped his cap, [...] He patted the boy on the shoulder. ‘Take it easy, son.’.
[US]H. Simmons Corner Boy 45: Man, I‘m poppin’ my cap. [Ibid.] 203: Man, I’m busting my cap.
[US]L. Rosten Captain Newman, M.D. 181: The Colonel has snapped his cap!
[US]S. Hayter Nice Girls Finish Last 91: After a hairy tour of Chechnya that left Reb a bit shell-shocked, he had been sent to Haiti, where he really snapped his cap.

2. (US Black) to appeal, to please.

[US]Pittsburgh Courier (PA) 25 Feb. 20/1: And that’s what snaps my cap about the East.