Green’s Dictionary of Slang

1. (US) the penis, thus sexual intercourse.

1963		T.I. Rubin Sweet Daddy 157: Gaff vagina; promiscuous women; sexual intercourse; [...] penis.
1998		J. O’Connor Salesman 294: But don’t try anythin’ smart or I’ll fuckin’ reef you round the gaff, I swear to Christ.

2. (US) the vagina, thus a promiscuous woman.

1963

see sense 1.

3. (US gay) a tight-fitting strip of rubber worn to disguise the male genitals, by female impersonators.

1965		K. Marlowe Mr Madam (1967) 20: Many female impersonators, or femme-mimics, work au naturel – except for a gaff.
1972		E. Newton Mother Camp 44: The stripper always wears a ‘gaff’ to conceal (strap back) his genitals.