noun A person who loses; one who fails to win or thrive.
In a two-horse race there is always one winner and one loser.
noun Something of poor quality.
noun A person who is frequently unsuccessful in life.
That guy is a born loser!
noun A contemptible or unfashionable person.
noun One who or that which loses something, such as extra weight, car keys, etc.
noun A losing proposition, one that is likely to lose or already has lost (such as a losing bet or, analogously, a predictably fruitless task or errand).
noun A person convicted of a crime, especially more than once.