Facial hair that grows on the underside of the chin. This style of beard is frequently sported by the overweight, in which case it technically resides on their second chin.
"Kids, take a page from the Underbeard King and get rid of your neckal hair,
`cause you're lookin' like Chewbacca did a face-plant into Nair."
~ excerpt from the traditional Ballad Of The Underbeard