A frube is a surfer who doesn't catch a wave for the whole time they are in the water.
Dude #1: The waves are so good! It's perfect!
Dude #2: Yeah dude! But that frube over there has been here for like 4 hours and still hasn't caught a wave!
Dude #1: Dude! What a frube!
Dude #2: Yeah dude! FRUBE!