Misery guts - Idioms by The Free Dictionary
An unhappy person who always complains. I don't like talking to Paul because he's such a misery guts and always squashes my good mood.
(a) ˈmisery guts (informal) a way to describe somebody who is never happy or who complains a lot: What’s the matter with you, misery guts? ♢ He used to be good fun, but he seems to be turning into an old misery guts.