To slap someone on the back, usually in a friendly or congratulatory gesture, or to give insincere praise.
From 'back' + 'slap,' emerging in American English around the early 20th century to describe both the physical action and the metaphorical act of false flattery.
The dual meaning of 'backslap' reveals something about American culture—it can be genuinely friendly OR completely fake, which is why 'backslapper' became slang for a politician or salesman who's all charm and no substance.
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