also: にやっとわらう
expression, godan verb (-u)
grin broadly; smirk
Describes a smug, knowing, or slightly unpleasant grin, often with a sense of self-satisfaction or hidden amusement.
彼はいたずらが成功して、ニヤッと笑った。
He grinned broadly when his prank succeeded.
秘密を知っているかのように
She smirked as if she knew a secret.