にへっ

adverb taking the 'to' particle

with a broad grin; with a smirk; with a cheeky smile

Mimetic adverb describing a wide, often self-satisfied or mischievous grin. Used with と or as a standalone interjection.

• 彼かれはにへっと笑わらって、秘ひ密みつを教おしえてくれた。

  He grinned broadly and told me the secret.