A person who is annoyingly knowledgeable or clever, usually in the phrase 'smart aleck.'
Possibly from 'Aleck' or 'Alex,' but the exact origin is uncertain. It may derive from a historical person known for being a know-it-all, though records are unclear. First recorded in mid-1800s American slang.
Nobody knows for sure who the original Aleck was, but his name became forever associated with annoying know-it-alls—which is a kind of immortality, even if not a pleasant one. It's one of those words where the human story is lost to time.
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