He's such a wet blanket.
그 사람은 분위기 깨는 데는 선수야.

John : May， do we need to buy anything else?
May : No， everything's set now. The food should be enough and we've got enough bottles of beer here. I guess we're okay.
John : How many people are we having tonight?
May : Well， I invited all of our friends including Tommy， so that'll make about 12.
John : You said you wouldn't invite Tommy because he's a party-pooper.
May : Yeah， I thought he would be no fun. But I felt bad afterwards. I mean he's such a wet blanket but he's not a bad guy.
John : I guess you're right.

* afterwards : 나중에
* party-pooper: 파티 등에서 분위기를 깨는 사람. pooper는 구어로 '바보， 얼간이'를 의미.
* wet blanket = A person who brinks everyone down : 다른 사람의 흥을 깨는 사람， 남의 행위에 찬물을 끼얹는 사람