An Etymological Dictionary of the German Language/Annotated/Eiche
Eiche, feminine, ‘oak, oak-tree,’ from the equivalent Middle High German eich, Old High German eih (hh), feminine; a term common to Teutonic, but by chance not recorded in Gothic (*aiks, feminine); compare Dutch eek (eik), Anglo-Saxon âc, feminine, English oak. In Iceland, where there are no trees, the old word eik, feminine, received the general meaning ‘tree’ (for a similar change of meaning see Esche, Föhre, Tanne; compare Greek δρῦς, ‘oak, tree (generally).’ The term aik- is peculiar to Teutonic; whether it is connected with Old Icelandic eikenn, adjective ‘wild,’ and with the Sanscrit root êj, ‘to shake,’ is undecided.