na-adjective, noun
killjoy; wet blanket; spoiling the fun
Refers to a person or thing that ruins an enjoyable atmosphere or dampens enthusiasm. Often used in phrases like 興ざましなことを言う (to say something that spoils the fun).
彼の一言でパーティーが興ざましになった。
His one remark killed the party mood.
興ざましなことを言わないでくれ。
Don't say things that spoil the fun.
興ざめ is the feeling of losing interest or enthusiasm (often used as a noun or suru-verb), while 興ざまし refers to the person or thing that causes that feeling.
Compound of 興 (interest, fun) and ざまし, the nominalized form of 冷ます (to cool down). Literally 'cooling down the fun'.