to have one's penis inside of a female. this can refer to having one's penis inside a female's vagina, mouth, etc.
I had a good time last night. After the party was over, I made sure I was in there, sticking it into all the girls who had followed me back to my apartment.
"In there like swimwear" is "in like Flynn" for a new millenium. Can you name a single Errol Flynn movie? But everyone knows (and loves) the Thong Song.