stick in the mud
Someone who is considered boring, often due to unpopular or outdated beliefs. Sally was tired of being called a stick in the mud by her friends just because she refused to drink alcohol. Just because I don't like roller coasters doesn't mean I'm a stick in a mud—I like lots of other fun things!
stick-in-the-mud
Fig. a dull and old-fashioned person. Don't be such an old stick-in-the-mud. some stick-in-the-mud objected to the kind of music we wanted to play in church.