Either soft, quiet laughs or the verb meaning to make such a gentle, restrained laugh.
Possibly from the word 'chuck' (to make a clucking sound), formed to imitate the sound; emerged in English around the 1700s as a specific word for quieter laughter.
Chuckles are the laugh of people trying not to laugh too hard—they're actually more common in real life than big belly laughs, but comedy writers rarely capture them because they're harder to describe on a page.
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