An Etymological Dictionary of the German Language/Teufel
Teufel, m., ‘devil, demon,’ from the equiv. MidHG. tiuvel (tievel), m., OHG. tiuval, tioval, m. (in the plur., neut. also); corresponding to OSax. diuƀal, Du. duivel, AS. deófol, equiv. to E. devil. The West Teut. words have apparently genuine Teut. sounds; on account, however, of the equiv. Goth. diabaúlus, Gr.-Lat. diabolus, it is certain that the word was borrowed. The early existence of the West Teut. word, attested by the permutation of LG. d to HG. t, can only be explained by the assumption that it was introduced into HG. in the 5th or 6th cent. through a Goth. medium (as also taufen, Pfaffe, Kirche, Heide, Samstag, and probably Engel), for the connection of the cognates with Gr.-Lat. (Ecclesiast.) diabolus cannot be doubted. The genuine HG. term for ‘evil spirit’ was Goth unhulþô, OHG. unholda, lit. ‘the demons.’