When a man perchs on the headboard of his bed, jerking off, trying to shoot a load on his significant other, while they walk around the bed waving to an imaginary crowd of people.
A possible abbreviation for "Just Fucking Killed", shouted by murder witnesses. If you want to send someone to their grave, be sure to take the person's life in a discreet area. In public, reactions will be unpredictable.
Murderer: "Now, to dispatch this fucker."
Witness: "What the actual fuck?!?! You JFKed that guy!!!"