When someone is asked to "be on best behavior," it means for them to behave in a polite and respectful manner. However, there are several antonyms that go against this positive behavior.
The first antonym is "be on your worst behavior." This means to behave negatively, such as being impolite, disrespectful, or causing trouble intentionally.
Another antonym for "be on best behavior" is "be on your wildest behavior," which means to behave recklessly or unpredictably.
Lastly, "be on your mediocre behavior" means to behave in an average or unexceptional way, without putting any extra effort into being polite or respectful.