adverb, adverb taking the 'to' particle, noun or participle which takes 'suru'
grinning broadly; smirking
Describes a wide, often unpleasant or self-satisfied grin. Typically used with 笑う or する, and often carries a negative or mocking nuance.
彼はいたずらが成功して、にたにた笑った。
He grinned broadly when his prank succeeded.
あの男はいつもにたにたしていて、感じが悪い。
That guy is always smirking and gives off a bad vibe.