when you think someone or something is stupid, you chin rub.
normally you look at them like they are stupid and rub your chin hinting that what they just said is dumb. but when you cannot see the person or do not feel like rubbing ur chin like you have a beard you say chin rub so people know thay wat they said was amazingly stupid and idiotic.
Sean: theres nothing to do
Mike: i knoww man....lets go to the park or something.
Cindy: wow mike, chin rub its raining u dumbshit.