Green’s Dictionary of Slang

cap n.7

[strained parallel to head n. (2c)]

(US black) the mouth, esp. as used in oral-genital sex; thus in phr. give (someone) some cap, to fellate, to perform cunnilingus.

[US] ‘The Fall’ in D. Wepman et al. Life (1976) 83: For lick and a lap from her mellow cap / The trick would fight a duel.
[US]R. Klein Jailhouse Jargon and Street Sl. [unpub. ms.].
[UK]J. Morton Lowspeak.