stick in the mud
Definition from Wiktionary, the free dictionary
Alternative forms 
- (idiomatic, pejorative) A person unwilling to participate in activities; a curmudgeon or party pooper.
- Have a little fun sometimes and don't be such a stick-in-the-mud.
- (idiomatic, pejorative) More generally, one who is slow, old-fashioned, or unprogressive; an old fogey.
- (person unwilling to participate): party pooper