* It's a misspelling. There might be a similar word that you're trying to remember.
* It's a regional term. It could be a word specific to a particular region or culture.
* It's a fictional term. It might be from a book, movie, or other creative work.
To help me figure out what you mean, could you please provide more context? For example:
* Where did you hear this word?
* What was the context in which you heard it?
* What do you think it might mean?
With more information, I might be able to help you understand what "argamannu" refers to.