Quoting my own message at the general message board at www.behindthename.com
Knight is a British occupational surname meaning "mounted warrior." The surname could also originally have been a nickname for a worker in a knight's household.
There's also a possibility that your surname is Irish, as there also exists the McKnight surname. So, your surname could also be a variant of it. Since the "Mc" means "son of" and "Knight" means "brave", the McKnight surname means "son of the brave."
So, your surname is either British or Irish.
I hope that helps.