If you're disapproving, you have a negative opinion about something. A disapproving frown from a favorite teacher is often enough to make a boisterous first grader sit down quietly.
Your mom's disapproving look at the dinner table might mean "Put your napkin in your lap," or "Don't talk about politics in front of Uncle Bill." Something that's disapproving conveys or expresses criticism or displeasure. Disapproving combines the prefix dis-, "the opposite of," with approving. from the Latin approbare, "to assent to or regard as good."