An Etymological Dictionary of the German Language/Annotated/Schlappe

by Friedrich Kluge

Schlappe (1.), feminine, ‘slipper,’ Modern High German only, from Low German slappe, winch is derived from Low German slapp, ‘loose.’

Schlappe (2.), feminine, ‘slap; discomfiture, defeat,’ Modern High German only, from Low German slappe; compare Middle English slappe, English slap; hence also in earlier Modern High German ‘slap in the face.’ From a High German *slapfe is derived Italian schiaffo, ‘slap in the face.’