Green’s Dictionary of Slang

1. an electronic drum machine.

1983		N.Y. Times 7 Dec. 24/1: In several songs, electronic drum machines or ‘beat boxes’ augment or replace the band’s drummer-percussionists.
1987		D. Hebdige Cut ’n’ Mix 139: Flash was also one of the first hip hop djs to work with a beat box: a machine that produces an electronic drum beat.
1995		Spin Dec. 120: L.L. Cool J, who reminds us just how thrilling one kid with a beat-box, Pumas, and an unlegislated imagination could be.
2000		W. Shaw Westsiders 50: Instead of a band it [i.e. hip-hop] substituted a beatbox and two turntables.

2. (US black) the making of drumming sounds by mouth; also as v.

1993		Source Dec. 80: I asked if he (Jarobi) rhymed. He beatboxed.
1994		Source Apr. 55: People know me for the beatbox.
2002		‘Touré’ Portable Promised Land (ms.) 155: We Words (My Favorite Things) [...] Ghetto blaster. Boomin system. Human beatbox.