To sneer means to smile in a cruel, mocking way that shows contempt or scorn for someone. When someone sneers, their face twists into an ugly expression, often with the upper lip curling up, that communicates “I think you're beneath me” without words. A bully might sneer at a classmate's drawing, silently communicating that they think it's worthless.

A sneer combines facial expression with attitude. It's a particular kind of unfriendly look that specifically shows disdain and superiority. Picture a villain in a movie looking down at the hero with that twisted half-smile that says, “You pathetic fool.” That's a sneer.

The word can also be used as a noun. You might say someone “gave a sneer” or “said it with a sneer in their voice.” Even tone of voice can sneer: when someone sarcastically says “Oh, how wonderful” with that contemptuous edge, they're sneering at you with words.

Sneering reveals something about the person doing it. They're trying to feel superior by making others feel small.