An Etymological Dictionary of the German Language/Annotated/eitel
eitel, adjective, ‘vain, idle, useless, void,’ from Middle High German îtel, adjective, ‘empty, vacant, vain, useless, fruitless, pure, unadulterated,’ Old High German îtal, ‘empty, vacant, vain, boastful’; corresponding to Old Saxon îdal, ‘empty, invalid,’ Dutch ijdel, Anglo-Saxon îdel, ‘empty, useless, worthless,’ English idle. The originally meaning of the adjective was probably ‘empty’; but if we accept ‘shining’ as the primary sense, it follows that the word is connected with Greek αἴθω, Sanscrit root idh, ‘to flame.’