What is lost? What is lost? Past tense and past participle of lose.Unable to find one's way: a lost child. No longer in the possession, care, or control of someone or something: a lost pen. No longer in existence; vanished or spent: lost youth. No longer known or practiced: a lost art. Beyond reach, communication, or influence: The expedition was lost to the world for two months. Not used to one's benefit or advantage: a lost opportunity. Having not been or unlikely to be won; unsuccessful: a lost battle; a lost cause. Beyond recovery or redemption; fallen or destroyed: a lost soul. Completely involved or absorbed; rapt: lost in thought. Bewildered or confused: I'm lost—can you start over? be lost on To have no effect or influence on: Her advice was lost on me. be lost on To be beyond the comprehension of: The lecture was lost on us.