To harm, hurt, injure, damage, spoil, or mar something — a transitive verb meaning to cause loss or impairment.
酒の飲み過ぎは健康を損なうおそれがある。
Drinking too much alcohol can harm your health.
彼の無責任な発言がチームの士気を損なった。
His irresponsible remarks damaged the team's morale.
環境汚染は生態系を損なう問題だ。
Environmental pollution is a problem that damages ecosystems.
A suffix after the -masu stem of a verb meaning 'to fail to do' or 'to miss doing' something (e.g. 聞き損なう = fail to hear).
会議の開始時間を聞き損なって、遅れてしまった。
I failed to hear the meeting start time and ended up being late.
映画の最初の場面を見損なってしまった。
I missed the opening scene of the movie.
メールを送り損なったために連絡が遅れた。
Because I failed to send the email, the communication was delayed.
A suffix used after the -masu stem meaning 'almost did' or 'came close to doing' something but did not actually do it (often used in constructions like 〜し損なうところだった).
危うく約束を守り損なうところだったが、なんとか間に合った。
I almost failed to keep my promise, but I managed to make it in time.
乗り損なうところだったが、ドアが閉まる前に飛び込んだ。
I almost missed the train, but I jumped in before the doors closed.