Snicker means:
* To laugh in a quiet, suppressed, or half-suppressed way, often at something that is considered to be inappropriate or amusingly embarrassing.
Think of a nervous giggle, or a laugh that someone tries to hide.
Here are some examples:
* "The children snickered behind their hands as the teacher tripped over the rug."
* "He snickered at her joke, though he didn't understand why it was funny."
* "She snickered at the embarrassing story he told, but quickly covered her mouth."