What it means
A know-it-all is that person who always has to be the smartest voice in the room. They've already read it, seen it, done it, and they'll jump in to correct you before you've even landed your sentence. It's not just knowing stuff, it's the smug little victory lap every time they prove you wrong. Even worse when they actually are right.
Usage examples
"Nobody wants the know-it-all on quiz night correcting the host's answers."
"He's such a know-it-all, he finished my sentence and then told me I was wrong."
"Don't ask Mia anything about films unless you've got ten minutes spare, she goes full know-it-all and starts correcting everybody."
"He turned one tiny question into a whole TED Talk, proper know-it-all behaviour."
Where it comes from
Know-it-all comes from the plain English phrase know it all, used for someone who acts like they know everything. It shows up in print by the 1800s as a mocking label, and the hyphenated form stuck as the standard noun for that unbearably certain type.
Other ways to say it
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