Definitions for know-it-all
Random House Webster's College Dictionary
a person who acts as though he or she had better knowledge or understanding than anyone else.
Origin of know-it-all:
someone who thinks he knows everything and refuses to accept advice or information from others
Kernerman English Learner's Dictionary
sb who thinks they know everything
You're such a know-it-all.
Someone who obnoxiously claims to be knowledgeable on a subject.
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.