口くちがイーってなる

expression, godan verb (-ru)

grin with closed teeth; make a tight-lipped grin

Colloquial expression describing a facial expression where the mouth stretches sideways into a tight grin, often when forcing a smile or holding back laughter. The 'イー' represents the sound of the mouth stretching.

• 変へんな顔かおをされたので、つい口くちがイーってなった。

  Someone made a weird face at me, so my mouth just stretched into a tight grin.

• 笑わらいをこらえようとして、口くちがイーってなってしまった。

  Trying to hold back laughter, my mouth ended up in a tight-lipped grin.