Green’s Dictionary of Slang

under n.

sexual intercourse.

1956

J. Curtis Look Long Upon a Monkey 81: Get yourself a good gutful, I always say. Worth all the under any tart’s got to dish out.

In phrases

bit of under (n.) [bit n.¹ ]

sexual intercourse.

1936		J. Curtis Gilt Kid 23: She’s got a bloke, a regular customer, get me, who pays the rent of the flat and floats in for a bit of under occasionally.
1937		J. Curtis There Ain’t No Justice 274: I ain’t going to pay you for a bit of under.

SE in slang uses

In phrases

on the under (adj.)

(US black) hidden, secret.

1992		Totally Insane ‘I Don’t Even Trip’ 🎵 People always wanna talk down on the under, man.
2003		Too $hort ‘Choosin’ 🎵 Just keep things on the under.