Nit-picking is looking for tiny faults and talking about them as if they were big. A nit is a very small egg of a louse, the kind that lives in hair. Long ago, picking nits meant going through hair one by one to find those eggs. The eggs are so small that you have to look too hard, and that picture stuck: a person who picks at every little mistake. We use it about people, comments, and meetings. It is not a kind word. If you say someone is nit-picking, you are saying they are being fussy about things that do not really matter. You will see a hyphen (nit-picking) or one word (nitpicking); same idea. Though actually, teachers and editors do need to catch small mistakes — so the word can be a bit unfair. The feeling it carries is “this is too small to make a fuss about.” People say it in everyday talk more than in careful writing.