To harm, hurt, injure, or damage something (e.g., health, reputation); to wreck or impair.
無理をして体を損ねてはいけない。
Don't harm your health by overdoing it.
彼の不用意な発言が会社の評判を損ねた。
His careless remark damaged the company's reputation.
誤った薬は身体を損ねる恐れがある。
Incorrect medication may harm the body.
As an auxiliary verb: to miss the chance to do something or fail to do what one ought to have done (used in constructions like '〜し損ねる').
電車に乗り損ねて遅刻した。
I missed the train and was late.
シャッターチャンスを撮り損ねてしまった。
I missed the photo opportunity.
大事な場面で言い損ねて後悔した。
I regretted failing to say something at an important moment.