* A misspelling: Perhaps you meant a similar word, like "Onigiri" (a Japanese rice ball).
* A regional term: It might be a word used in a specific region or community, but not widely known.
* A made-up word: It could be a fictional term from a book, game, or other creative work.
To understand what "onisan" means, please provide more context:
* Where did you encounter this word?
* What was the surrounding text or conversation?
With more information, I can help you figure out its meaning.