(verb)- to be overly cheerfully deliberately misleading.
"The little bastard grin fucked me when he told me he'd finished the project."
grin fuck (verb) - a sales technique in which the salesman smiles profusely while delivering bad news. Since 80% of human communication is based on body language alone, the simple deception of grin fucking is often enough to prevent the victim from fully conceptualizing the bad news until later, after they've already agreed to the deal.
Note: the best defense against grin fucking is to call attention to it by casually asking the salesman why they are smiling.
Salesman <while grin fucking the hell out of you> We had to order a part from overseas, so the invoice came in $500 over the original estimate.
You <after sipping your coffee>: Why are you smiling? That sounds like bad news to me.
Salesman <suddenly befuddled>: Oh... no reason. I guess I'm just in a good mood today.
You: Okay, well, since I never approved the additional costs, what discounts can you provide to bring us back closer to the original estimate?