niya-niya
A knowing, slightly mischievous grin. Not a warm smile — more like you're amused by a secret or something sneaky.
💡Reading a funny text in public — 'Stop niya-niya at your phone, people are staring.'
スマホを見てにやにやしている。
He's niya-niya grinning at his phone.
She was ___ while reading the secret love letter.
ラブレターを読みながら○○していた。