Someone who acts like they're very clever or thinks they know everything, often in an annoying way; a show-off.
From 'smart' (meaning clever or witty) plus the -y suffix to create an informal, slightly derogatory noun. Common in American English from the early 1900s, often used in the phrase 'smarty-pants.'
The candy 'Smarties' was named to suggest the candy makes you smarter—but 'smarty' as an insult is older and originally meant someone who was smartly dressed or quick-witted, which got twisted into mockery over time!
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