Snicker (verb)

Laugh quietly or to oneself in a half-suppressed or partly disguised manner.

Origin:

probably imitative of the sound of suppressed laughter.

Examples:

1. She couldn't help but snicker at the silly joke.
2. He couldn't help but snicker when she tripped.
3. They all snickered behind her back.
4. She snickered at the irony of the situation.
5. He snickered at the ridiculous idea.