負ける：まける： lose a game or fight or struggle.
参った：まいった： (spoken aloud) to say uncle. "no mas!"
敗北：はいぼく： a loss of a game or war (noun)
失敗：しっぱい： any failure! test, your plan fails, no one comes to your party, your business goes bankrupt.
失う：うしなう： to miss out on something (but not because of an opponent). Like you slept too late and missed your TV show.
不合格：ふごうかく： unacceptable! (used with schoolwork, but also work reviews, or 'this workmanship is unacceptable' or 'Son your room is not acceptably clean.')
落第：らくだい： to fail a test