Green’s Dictionary of Slang

an excl. of encouragement, usu. in the context of a musical performance, whether live or created by a DJ playing records/tapes.

1970		Current Sl. V:2.
1977		C. McFadden Serial 20: A mass of writhing bodies, from which issued an occasional cryptic cry of ‘Get down!’.
1980		Eble Campus Sl. Spring 3: get down – to go ahead; to try hard [...] Oh, get down!
1988		Viz Oct./Nov. 27: Hey get down! Cos I am the baddest bobby in town!
2002		‘Touré’ Portable Promised Land (ms.) 154: We Words (My Favorite Things) [...] Raise the roof! Get down! Git busy! Go berserk! Go head! Go head!