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.
From the tendency of thongs and other tightfitting swimwear to become wedged where the sun never shines
"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.
"Make friends with her friends and you'll be in there like swimwear."
from The Player's Rulebook