If you lose a contest, a fight, or an argument, you do not succeed because someone does better than you and defeats you. A C Milan lost the Italian Cup Final The government lost the argument over the pace of reform No one likes to be on the losing side
suffer the loss of a person through death or removal; "She lost her husband in the war"; "The couple that wanted to adopt the child lost her when the biological parents claimed her"
If you lose something, you do not know where it is, for example because you have forgotten where you put it. I lost my keys I had to go back for my checkup; they'd lost my X-rays
If you lose blood or fluid from your body, it leaves your body so that you have less of it. During fever a large quantity of fluid is lost in perspiration