Green’s Dictionary of Slang

(US) to perform fellatio or cunnilingus.

1919		Transcript Foster Inq. in L.R. Murphy Perverts by Official Order (1989) 42: Arnold had to explain again that ‘getting down on someone’ meant ‘he gets down on him and sucks another man’s cock’.
1930		H. Miller Letters to Emil 51: Start off by letting her get down on me.
1934		‘J.M. Hall’ Anecdota Americana 177: ‘Eat fur pie’ [...] otherwise known as ‘Cleaning the kitchen,’ ‘Gobbling the gravy,’ ‘Getting down on it,’ scientifically cunnilingus.
1965		Guild Dict. Homosexual Terms 19: get down (on) (v.): To fellate or cunnilingue.
1984		C. White Life and Times of Little Richard 152: She got back down with me.