To penetrate sexually.
Paul: “Did you make in with that drunk girl last night?”
John: “No, just out…”
Make In: define #2
kissing, with lips not generally touching, only tongues fiercely and feverishly licking each other and swirling around for an elongated period of time.
– do not confuse with make out or making out –
We made in and made out in the rain.