1To harm, damage, or spoil a person or thing, worsening its state.
過度なストレスは健康を損なう。
Excessive stress harms your health.
一度の失言が信頼を損なうこともある。
A single slip of the tongue can sometimes damage someone's trust.
開発によって自然環境を損なってはいけない。
Development must not damage the natural environment.
2Attached to the -masu stem of a verb as ～し損なう, meaning to fail to do or to miss the chance to do something.
大事な連絡を聞き損なって、後で慌てて確認した。
I failed to catch an important message and had to scramble to check it later.
バスに乗り損なって、会社に遅刻してしまった。
I missed the bus and ended up being late for work.
メールを送り損なったせいで、返事が遅れてしまった。
Because I failed to send the email, my reply ended up being delayed.
3Attached to the -masu stem of a verb to mean almost doing something, or coming close to doing it (but not actually doing so).
あと少しで終電に乗り損なうところだったが、なんとか間に合った。
I almost missed the last train, but I just barely made it in time.
大事な書類を提出し損なうところだったが、締め切り直前に気づいた。
I nearly failed to submit an important document, but I noticed just before the deadline.