In Japanese, にやり (niyari) is the onomatopoeia for a sly, scheming grin, the written equivalent of a character smirking with a secret, and it has long appeared as a sound effect in manga and anime. Online creators adopted the word for short reaction clips and meme edits, where the knowing smirk signals that a plan is already in motion. Soundboard versions of the Niyari grin now show up in prank videos, gaming highlights and anime compilations, giving editors a wordless way to telegraph mischief.