Opposite Effect
noun
効果 is "effect," so the opposite of said effect is the opposite effect. Whoops!
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. Will help to know 効果 as well.
彼女に焼き餅を妬かせようとして他の女にキスをしたら、全く逆効果になってしまった。
I kissed that other girl to make my girlfriend jealous, but it really backfired.