(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.
2004, Princeton Review, Know It All! Grades 9-12 Reading, The Princeton Review, →ISBN, page 22:
The same holds true for supporting details—they help you to learn more about a topic. Be a know it all by sharing these details and supporting details with friends.
2005, Pansie Hart Flood, Tiger Turcotte Takes on the Know-it-all, Carolrhoda Books, →ISBN:
She's a mean, bossy know-it-all.
2012, Joe Ike, How do I relate with a “know-it-all”?, xBusinessServices Corp., page 60:
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?