1.
laughing at a memory; chuckling to oneself
suddenly laughing while remembering something funny, often catching others off guard
Compounds
思い出す (to remember) + 笑い (laughter)
Usage
This often has a slightly embarrassing or socially awkward connotation, as laughing suddenly in public without an apparent reason can seem strange to others.
思い出し笑い in public places like trains can be socially awkward, which is why it's often mentioned with ~してしまう (to do accidentally).