An Etymological Dictionary of the German Language/foltern
foltern, vb., ‘to put to the rack, torture,’ from late MidHG. vultern, ‘to put on the rack.’ Akin to Folter, ‘rack,’ early ModHG only. of obscure origin. It is most frequently considered to be partly translated and partly borrowed from MidLat. pulletrus, poledrus, prop. ‘colt,’ which signifies ‘rack’ in Span. and Port. (potro), “like Lat. equuleus from equus, because it bore some resemblance to a horse.” MidLat. poledrum is derived again from Gr. πῶλος, ‘foal.’ “The wooden horse and the wooden ass — frames with a sharp-edged back, upon which the delinquents were compelled to ride — were favourite instruments of torture.”