"Cast" means to throw, and when you "cast something back in someone's teeth", you are throwing words at them, often reminding them of something they themselves once said: reminding them of a time they were wrong, generally. This would happen in a bitter fight. Example: The wife turned on her husband and cried out, "You were the one who said we should buy this house. You said it was a good investment!". The husband growled, "And you agreed. You have no right to cast that in my teeth!"

'cast in someone’s teeth' is an old fashioned idiom meaning to upbraid or reproach a person; to throw back at a person something he has said or done. Some say the phrase, popular in Shakespeare’s time, is an allusion to knocking someone’s teeth out by casting stones. The current expression is " to throw in someone’s face." Example: I know I should have checked the time for your arrival, but there is no need to keep throwing it in my face.

May be it means to test someone.