Green’s Dictionary of Slang

(US) to take a narcotic or other recreational drug, usu. heroin.

1952		Hal Ellson Golden Spike 92: ‘Where you going?’ [...] ‘To get down again.’.
1972		O. Hawkins Ghetto Sketches 69: I fell short on my hustle one night, Jones was on me, so I came home and stole the baby’s milk money to get down with.
1982		M. Starks Cocaine Fiends and Reefer Madness [picture caption for ‘High on the Range’ (1929)] 32: Young scamp Dave getting down with ‘locoweed’ – probably the first time in films that marijuana smoking was shown.
1989		(con. 1930s) Courtwright & Des Jarlais Addicts Who Survived 53: If somebody came up and said, ‘Look, man, I got some pot, man. Y’all want to get down?’ we’d say, ‘Yeah, man’.
2001		ONDCP Street Terms 10: Get down — To inject a drug.