It's possible that:
* It's a misspelling: Perhaps you meant another word like "mummer," "muser," or "mouser."
* It's a regional dialect word: Some words are specific to certain regions or communities.
* It's a made-up word: It's possible it's a nickname, a slang term, or a word invented for a specific context.
To help me understand what you're looking for, please provide more context:
* Where did you encounter the word?
* What was the surrounding sentence or conversation?
* What do you think the word might mean?
With more information, I might be able to figure out what "mumser" means.