be on best behaviour

be on your best behaviour

(British & Australian) also be on your best behavior (American & Australian)

to behave very well, usually because you are in an important or formal situation Now children, I want you all to be on your best behaviour when grandma arrives.

Common Names:

Name	Pronounced	Usage
Durham	['dʌrəm]
Kobina	-	Western African, Akan
Alejandro	ah-le-KHAHN-dro	Spanish
Parthenia	par-THEE-nee-ə (English)	Greek Mythology
Bertha	['bə:θə]
Bamber	['bæmbə]