What is the proper diffination of mis?

"Mis" is not a word with a stand-alone definition. It's a prefix that is added to the beginning of words to give them a negative or wrong meaning.

Here are some examples:

* Misunderstand: to understand something incorrectly.

* Misplace: to put something in the wrong place.

* Misinform: to give someone wrong information.

* Misbehave: to behave badly.

So, "mis" itself doesn't have a definition, but it changes the meaning of the word it's attached to.