Green’s Dictionary of Slang

have it in v.

to have sexual intercourse.

[UK]Lustful Memoirs of a Young and Passionated Girl 29: I have had it in Julia twice today.
[UK]G.F. Newman Sir, You Bastard 12: ‘Ot, innit?’ the weather. ’Ad it in lately’, sex.
[UK](con. WWII) B. Aldiss Soldier Erect 14: Sod it, I had to have it in tonight.
[UK]Partridge DSUE (8th edn) 537: late C.19–20.