(verb) To have sexual intercourse with your girlfriend/wife/significant other while she is on her period. "Hard" being the state of the penis and "In The Paint" referring to the red-ish period blood of the female.
J: Yo man, let me tell you about last night!
Bob: What happened bro??!!
J: I totally went Hard In The Paint with Rebekah! It was by far the slickest ride I've ever took in the Gentlemen's Pleasure Garden!