A Jill sandwich is a zombie delicacy, prepared by crushing S.T.A.R.S. member Jill Valentine (of Resident Evil fame) in a room with a slowly-descending ceiling. When Jill is reduced to a nice, squishy pulp, zombies spread the resulting pulp between two slices of bread and enjoy a nice snack. Alternatively, if zombies are out of bread, they can proceed to lick the Jill pulp off the floor.
Notorious quote from the original Resident Evil for PlayStation where Barry Burton provides his STARS partner Jill Valentine with some comic shortly after being rescued from a crushing death. Also see master of unlocking.
"That was too close; you were almost a Jill sandwich."