adverb, adverb taking the 'to' particle
with a meaningful grin; suggestively; sneeringly
Describes a brief, knowing, or suggestive smile, often with a hint of satisfaction, mischief, or scorn. Typically used with 笑う or する.
彼は秘密を知っているかのようにニヤリと笑った。
He grinned meaningfully as if he knew the secret.
犯人は
The suspect just smirked at the detective's question.
Onomatopoeic or mimetic word; the exact derivation is uncertain.