adverb taking the 'to' particle
smirkingly; with a broad grin
Describes a smug, self-satisfied, or meaningful smirk, often with the particle と.
彼はいたずらが成功して、にやっと笑った。
He grinned broadly when his prank succeeded.
彼女は何か企んでいるように、にやっと口元をゆがめた。
She twisted her mouth into a smirk, as if she were plotting something.