pat on the back
pat on the back (1) {v. phr.}
1. To clap lightly on the back in support, encouragement, or praise.
The coach patted the player on the back and said a few encouraging words.
2. To make your support or encouragement for (someone) felt; praise.
After he won the game, everyone patted him on the back for days.
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Source: A Dictionary of American Idioms