What does demonsterous mean?

"Demonsterous" is not a standard English word. It's likely a misspelling of "demonstrative", which means:

* Showing feelings openly and directly: This can be applied to someone's personality, like "She's a very demonstrative person and shows her love openly."

* Clearly showing or proving something: This can be used to describe evidence, like "The witness provided demonstrative evidence of the suspect's guilt."

If you meant something else, please provide more context!