1. [very common] To fail. A program loses when it encounters an exceptional condition or fails to work in the expected manner.
2. To be exceptionally unesthetic or crocky.
3. Of people, to be obnoxious or unusually stupid (as opposed to ignorant). See also deserves to lose.
4. n. Refers to something that is losing, especially in the phrases “That's a lose!” and “What a lose!”