A fictional (sexually transmitted disease) that causes extreme discomfort when urinating.
Internal crabs do not exist BUT watch your buddy's face when you tell him he has them.
Man #1: Dude, I boned this chick the other night without a (jimmy) and now it burns like crazy when I (piss).
Man #2: Maybe it's the internal crabs.
Man #1: The what?
Man #2: Maybe she had (crabs). They probably jumped out of your pubes and crawled up into your (dick hole) while you were asleep.
Man #1: You're full of shit!
Man #2: Dude, the internal crabs. I'm telling you. They're trying to chew their way back out of the tip of your dick. That's why your piss burns.
Man#1: Jesus, that skank
gave me the internal crabs!
Man #2: Get thee to a doctor, bro. They're fatal.