adverb taking the 'to' particle
smirkingly; smugly; with a grin
Describes a self-satisfied, knowing, or slightly unpleasant smile, often used with 笑う or する.
彼はこちらの失敗を見て、ニタリと笑った。
He smirked when he saw my mistake.
犯人はニタリとした表情で黙っていた。
The culprit stayed silent with a smug look on his face.
Mimetic word; the exact origin is uncertain, but it is associated with the sound or appearance of a self-satisfied smile.