クスッと

adverb

with a quiet chuckle; unintentionally letting out a laugh

Describes a small, suppressed laugh that slips out unintentionally. Often used with verbs like 笑う or こぼす.

• 彼かれの冗じょう談だんにクスッと笑わらってしまった。

  I unintentionally let out a quiet laugh at his joke.

• その話はなしを聞きいて、思おもわずクスッとこぼした。

  Hearing that story, I couldn't help but let out a little chuckle.