A person who gives insincere praise or false flattery, or someone who greets people with overly friendly back-slapping.
From 'backslap' + '-er' (one who does the action), coined in 20th century American English to describe a stereotypical glad-handing salesman or politician.
Mark Twain and other 19th-century writers used 'backslapper' to satirize American politicians—it perfectly captures that type of person who's friends with everyone at the party but remembers nobody the next day.
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