Green’s Dictionary of Slang

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

a.1963		‘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.
1983		R. Klein Jailhouse Jargon and Street Sl. [unpub. ms.].
1989		J. Morton Lowspeak.