When a guy ejaculates in a girls vagina, keeping all the cream inside, and clean on the outside. In the song, said multiple times Cream on the inside, Clean on the outside, Ice cream paint job.
Justin: I was fucking my girl last night, and i gave her a Ice Cream Paint Job.
Eric: Cream on the inside clean on the outside?
Justin: Hell yeah man, Luckily she was on Birth Control.