A noun/na-adjective meaning the spoiling of enjoyment or loss of interest — a dampener or turn-off that kills the mood.
彼の冷たい一言で一同は興ざめした。
One cold remark from him put everyone off.
せっかくの会合が遅刻者の失礼な態度で興ざめになった。
The long-awaited meeting was spoiled by the rude behavior of a latecomer.
そんな細かい指摘ばかりでは興ざめだよ。
Constant nitpicking really kills the mood.