ETYMOLOGY
from flap (to move up and down) + sauce (n.)
EXAMPLE
“…hath nowe brought (to howse) nowe hathe this glutton. i. this flappe sawce (the thyng) that he may plentuously swallowe downe hole…”
From: Gulielmus Gnapheus’ Comedye of Acolastus
Translated by John Palsgrave, 1540
Lexicophilia 