Opposite Effect
Counterproductive
noun, の adjective
効果 means "effect," so the opposite of said effect is the opposite effect. Or in other terms, something counterproductive.
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.
それ、逆効果じゃない？
Isn't that counterproductive?
励ますつもりが、逆効果になってしまった。
I wanted to cheer him up, but it had the opposite effect.
胃ぶくろを満たしてハートをつかむはずが、逆効果だった。
I expected to capture his heart by filling his stomach, but it backfired.