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.

• "on one's best behavior" meaning in Chinese: 行为检点；规矩
• "on one's best behavior" meaning in Japanese: 《be ～》おとなしくしている、神妙にしている、よそ行きの態度を取っている、行儀よく振る舞う、行儀良くしておく