adverb, adverb taking the 'to' particle, noun or participle which takes 'suru'
with a grin; smilingly; beaming
Describes a cheerful, gentle, sustained smile, often with a friendly or contented feeling. Can be used adverbially (ニコニコ笑う) or as a する-verb (ニコニコしている).
彼はいつもニコニコしている。
He is always smiling cheerfully.
赤ちゃんがニコニコ笑
The baby grinned happily.