have it up, v. — Green’s Dictionary of Slang

have it up v.

to have sexual intercourse.

1980		Maledicta IV:2 (Winter) 196: The terms used for copulating and the sexual act [...] very largely consist of variations on the theme of vague ideas such as have it off, up or away.
1984		Partridge DSUE (8th edn) 537: late C.19–20.