be on one's best behaviour BrE
be on one's best behavior AmE
be on your best behaviour

You know, you'll have to sort of be on your best behaviour!

"I want you to be on your best behaviour while Rex is away."

"Thaf s all very well, but you'd better be on your best behaviour.

So be on your best behaviour.

And I want you all to be on your best behaviour, because if you aren't, I might get a little angry.

But you will take care to be on your best behaviour for the remainder of your stay here, and I also strongly suggest that you leave these waters soon.

Mind your Ps and Qs is an English expression meaning "mind your manners", "mind your language", "be on your best behaviour" or similar.

You could behave like a pig and they would simply think how sweet and original you were, whereas if you were wildly in love you would be on your best behaviour, nervously uncertain and consequently lacking in charm.

You always want to be on your best behavior when the boss shows up.

You know, you'll have to sort of be on your best behaviour!

I trust you will all be on your best behavior."

You'll just all have to be on your best behavior."

"You're supposed to be on your best behavior when you visit a place, and not trash it."

"You must be on your best behavior when you meet her."

"I want you to be on your best behaviour while Rex is away."

Are you telling me you've been on your best behavior since I last saw you?

I'll ask you to be on your best behavior."

"Before you go," she said, "I want to warn the both of you to be on your best behavior.

But you must not gloat and you must be on your best behavior.

"Aye," said Mick, "so you be on your best behavior, hear?"

"Better be on your best behavior, Major."

"I can get you into certain nonrestricted areas, but you've got to be on your best behavior because you're going to be noticed."

"Thaf s all very well, but you'd better be on your best behaviour.

So be on your best behaviour.

You will be on your best behavior this day, or I shall see you flogged for your indiscretions.

If Guinan was kind enough to let you in, then it behooves you to be on your best behavior rather than running about."

And I want you all to be on your best behaviour, because if you aren't, I might get a little angry.

"You're going to have to be on your best behavior and bite your tongue at this ranch, Doc," Terry said to her.

'Tis very important for you to attend this dinner, and you must be on your best behavior."

"Be on your best behavior," Brount warned.

"Now, Ralphy, you be on your best behavior in the lobby while we wait for Aunt Roberta."