To give a small, suppressed chuckle or giggle; to let out a short, quiet laugh.
彼の冗談に思わずクスッと笑ってしまった。
I couldn't help but chuckle softly at his joke.
会話の合間に彼女がクスッと笑って、場の空気が和んだ。
She let out a quiet chuckle during the conversation, which eased the mood.
小さなミスを見て先生がクスッと笑った。
The teacher chuckled softly at the small mistake.