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