A buttwink is when a person flexes their
anal sphincter, causing their anus to visibly contract, effectively "winking" with their asshole. Buttwinking can be performed as an obscene gesture, or in a sexual context it can be either a sign of affection or a greeting.
To buttwink, simply use the same muscles used to defecate to flex
your anus.
This act should not be confused with mooning (merely exposing
your anus) since buttwinking is considerably more rude.
As Sensei Doug walked to his dojo, the rude
employees of a strip mall store lowered their drawers, pointed their behinds towards the window, and
contracted their assholes to buttwink at the passing
martial artist.