An Etymological Dictionary of the German Language/Annotated/Rappe
Rappe (1.), masculine, ‘black horse,’ Modern High German only in this sense, which is a figurative use of Middle High German rappe, ‘raven,’ the variant of Middle High German rabe (Alemannian rap, ‘raven’). Old High German *rappo is wanting; it would be related to rabo like *knappo, ‘squire,’ to knabo, ‘boy.’ See Rappen.
Rappe (2.), feminine, ‘malanders,’ from Middle High German rappe, rapfe, feminine, ‘itch, scab’; allied to Dutch rappig, ‘scabby.’ The root is seen in Old High German rapfen, ‘to harden (of wounds), form a scab,’ and in râffi. From High German the equivalent French râpes, plural, is formed.
Rappe (3.), feminine, ‘rasp,’ Modern High German only, from the equivalent French râpe, which again corresponds to Old High German raspôn, ‘to sweep off,’ Middle High German raspeln. See raffen, Rapp, and Raspe.