wet blanket

Short definition from Wiktionary

Noun

1. 1
   A person who takes the fun out of a situation or activity, as by pessimism, demands, dullness, etc.
2. 2
   A timid person; one who lacks confidence; a wimp.
3. 3
   Used other than figuratively or idiomatically: see wet, blanket.

Verb

1. 1
   Alternative form of wet-blanket.
2. 2
   To disparage, belittle, or remove the satisfaction and joy from; to act as a wet blanket towards.