When you are "fucked", or taken advantage of, by a lie told from a grinning mouth.
The smiling used car salesman using lies, convinces a customer to buy a car that breaks down a week later. The customer was grinfucked.
To fuck up someone's grin when they are grinning like no other
Joe came in grinning today so I told him I fucked his girlfriend. Joe became grinfucked.