on one's best behavior

Idioms  

1. Also, on one's good behavior. Very polite, as in Mother told the children to be on their best behavior during Grandma's visit, or The whole staff were on their good behavior while the client inspected the premises. [Late 1600s]

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Example Sentences

Typical first-date advice is to be on one’s best behavior.

From Washington Post

The best holidays for joke-writing are Thanksgiving and New Year’s Eve because you get the most people together for the longest, so the idea of being on one’s best behavior quickly erodes, revealing easily observable human nature.

From New York Times

The book quotes one acquaintance as saying she “had the knack of putting one on one’s best behavior.”

From New York Times