also: にいっと
adverb, noun or participle which takes 'suru'
with a grin; grinningly
Mimetic adverb describing a brief, deliberate grin or smirk. Often used with 笑う or する.
彼はいたずらっぽくニッと笑った。
He grinned mischievously.
写真を撮るとき、彼女は
When taking the photo, she gave a quick grin to the camera.