Penthouse crabs are lice. They get their name from crabs, which are mainly found in the pubic region, who migrate, or "move on up" to the head, the "penthouse" of the body.
Did you see Brittney Spears scratching her head on Dateline the other night? It looked like she definately got a case of the penthouse crabs.
by Squirrely June 19, 2006