To snicker is to laugh in a mean or disrespectful way, often expressing superiority. We might snicker at a bully who walks directly into a closed door.
You can also use this word's near-synonym, snigger, to describe a short snort of a laugh or a scornful sound. You know your stand-up comedy act isn't going well when you don't even get snickers from your audience. A secondary meaning of snicker is horse-related: to whinny. It's actually from this use that the Snickers candy bar gets its name. In 1930, the Mars family named it after their favorite horse — Snickers.