What does butthurt in the first degree mean?

"Butthurt" is a slang term used commonly on forums when a poster has a negative reaction to something said in a thread. Example:

Poster 1: "LOL 9/11"

Butthurt Guy: "That's rude, people died."

Poster 2: "That guy's got a case of the butthurt."

Poster 1: "LET THE BUTTHURT FLOW THROUGH YOU"