にへら

Meanings

adverb, adverb taking the 'to' particle

grinning goofily; with a slack, silly smile

onomatopoeic or mimetic wordcolloquial

Colloquial mimetic word describing a loose, silly, or absent-minded grin. Often used with と or as an adverb. Blended from にやにや (smirk) and へらへら (grin foolishly).

彼かれは何なにがおかしいのか、にへらと笑わらっている。
He's grinning goofily, I don't know what's so funny.
にへらにへらしながら、ぼんやりテレビを見みていた。
He was watching TV absent-mindedly with a slack grin on his face.