expression, godan verb (-mu)
smile; crack a smile
Describes the mouth relaxing into a smile, often involuntarily or when one is amused. The subject is usually the person whose mouth relaxes.
彼の冗談を聞いて、思わず口元が緩んだ。
Hearing his joke, I couldn't help but smile.
赤ちゃんを見
When I look at the baby, my mouth naturally relaxes into a smile.