"Slap Jeffrey" is an objective in the sidequest Winter Is A Bloody Business from Tiny Tina's Assault on Dragon Keep DLC in Borderlands 2
Walkthrough[edit]
Wait for Jeffrey to stand up from his throne. Just hit him with anything: shoot him, grenade him, melee him. He goes down very, very, very, very quickly.
Now just melee him a few times. Anything additional is just good fun.