(chiefly US, derogatory) Someone who claims to be knowledgeable or an expert in something, obnoxiously dismissing the opinions, advice, suggestions, etc. of others.
We had it all figured out, but this know-it-all marched in with "the correct way of solving it", leaving our experiment in shambles.
Have you ever gone shopping with a Know-It-All girlfriend and she insists, without your consent or consulting you first, to choose your outfits for you?
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