An Etymological Dictionary of the German Language/eitel
eitel, adj., ‘vain, idle, useless, void,’ from MidHG. îtel, adj., ‘empty, vacant, vain, useless, fruitless, pure, unadulterated,’ OHG. îtal, ‘empty, vacant, vain, boastful’; corresponding to OSax. îdal, ‘empty, invalid,’ Du. ijdel, AS. îdel, ‘empty, useless, worthless,’ E. idle. The orig. meaning of the adj. was probably ‘empty’; but if we accept ‘shining’ as the primary sense, it follows that the word is connected with Gr. αἴθω, Sans. root idh, ‘to flame.’