on one's best behavior meaning

[American idiom]
being as polite as possible.
• When we went out, the children were on their best behavior.
• I try to be on my best behavior all the time.

[American slang]
being as polite as possible.
   When we went out, the children were on their best behavior.
  I try to be on my best behavior all the time.

• at one's best:    [American idiom]in ...
• on one's good behavior:    [American slang]ad ...
• as best one can:    As well as one can ...