A play on the English writer J.K. Rowling's (writer of the Harry Potter Series) name. It usually means "Just kidding, and I am laughing aloud at the same time" or just "I am currently laughing aloud".
Frank: Dude you're such an asshole sometimes, you know Sarah and I broke up last week.
Tongue Lolling is used to describe when a girl in a hentai is fucked sillyto the point her eyes roll up and her tongue hangs out like a dog. It's pretty common in hentai to see a girl lolling her tongue out as some guy fucks her brains out.