adverb, adverb taking the 'to' particle, noun or participle which takes 'suru'
grinning; smirking
Describes a broad, often self-satisfied or suggestive grin, usually without laughing out loud. Can carry a negative nuance of smirking or leering, but also used for a simple happy grin.
彼は何がおかしいのか、一人でにやにやしている。
He's grinning to himself, I wonder what's so funny.
いたずらを思いついた子供のように、彼女はにやにやと笑った。
She grinned like a child who had thought of a prank.