Green’s Dictionary of Slang

to have sexual intercourse.

1904		Lustful Memoirs of a Young and Passionated Girl 29: I have had it in Julia twice today.
1970		G.F. Newman Sir, You Bastard 12: ‘Ot, innit?’ the weather. ’Ad it in lately’, sex.
1971		(con. WWII) B. Aldiss Soldier Erect 14: Sod it, I had to have it in tonight.
1984		Partridge DSUE (8th edn) 537: late C.19–20.