What does back-slap mean in this sentence?
Back-slap someone in Korea and they will be offended.
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Literally, to slap someone on the back. In the US, it's a gesture of conviviality or congratulations.
However, to call someone a "back-slapper" is something of an insult. There's a strong connotation that the conviviality or congratulations is insincere. Politicians are often called "back-slappers" (or "glad-handers", meaning a person prone to shaking a strangers' hands and grinning inanely, a similar dig).