To laugh nervously, often at something that you feel you should not be laughing at. (Cambridge Dictionary)
If someone titters, they give a short nervous laugh, especially when they are embarrassed about something. (Collins Dictionary)
To laugh in a nervous, affected, or partly suppressed manner: Giggle, snicker. (Merriam – Webster)
Give a short, half-suppressed laugh; giggle. (Oxford Dictionaries)
Titter cartoon and comics.