Slap: v. tr. and pron. (or bawl, baffle; baffle; baffle -se -). To eat, to eat greedily, with greed, a lot, to devour.
We had a hell of a meal.
This word probably comes from onomatopoeia baf which reflects the sound of the lips or the swelling of the cheeks. The same mimicry is used to signify contempt from which to flout, also derived from onomatopoeia baf by Provencal baffle : mock.
We can also assume that the Germanic radical baf (lip) is at the origin of the two verbs of bawling and baffling, but does it not itself come from the onomatopoeia in question?