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Someone who is considered boring, often due to unpopular or outdated beliefs.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!
If you call someone a stick-in-the-mud, you mean they are old-fashioned or boring and do not like doing new things or having fun. I felt sorry for him because he obviously wanted to enjoy himself but was married to a real stick-in-the-mud. Call me a stick-in-the-mud, but I just prefer the more traditional ways of doing things. Note: You can use stick-in-the-mud before a noun. He's going to let an opportunity pass him by, with his stick-in-the-mud attitude. Note: This expression is used to show disapproval.