1. a sack of sadness
2. a character from the 80's childrens cartoon "Raggy Dolls"
fuck you're a sad sack
don't be such a sad sack or i'll punch you in the mouth
A person who's only role in a friendship is to listen to the other friend whine and complain about their troubles.
Lisa only calls me when she wants to complain about how bad her life is. I'm tired of being her sad sack.