Green’s Dictionary of Slang

cap n.6

[they ‘cap’ the body]
(US black)

1. [1950s+] the mind; implied in snap one’s cap

2. [1990s+] the top of the head, the cranium; implied at peel caps under peel v.

In phrases

out of one’s cap

[2000s] (N.Z.) having lost one’s temper.

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

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