An Etymological Dictionary of the German Language/Kaiser
Kaiser, m., ‘emperor,’ from the equiv. MidHG. keiser, OHG. keisar; corresponding to AS. câsêre, Goth. kaisar. The ai of the ModHG. orthography originated in the Bav. and Aust. chancery of Maximilian I., in which the MidHG. ei necessarily became ai (according to the lexicographer Helvig, A.D. 1620, Meissen Sax. Keiser was Bohem.-Bav. Kayser). The ae of Lat. Caesar, upon which the word is based, cannot be made responsible for the ModHG. ai. Moreover, the relation of Lat. ai to Goth.-Teut. ai is not explained. The Romans, it is true, used ae for ai in Teut. words, comp. Lat. gaesum, under Ger; yet the use of Teut. ê to represent ae in Lat. Graecus (Goth. Krêks, OHG. Chriah, ‘Greek’) is opposed to this. At the same period as the adoption of the names Grieche and Römer (Goth. Rûmôneis), i.e., the beginning of our era, the Teutons must have borrowed the Lat. term, connecting it chiefly with Caius Julius Cæsar (similarly the Slavs use the name Karl der Große of Charlemagne, in the sense of ‘king’; OSlav. kraljǐ, Russ. korolǐ, whence Lith. karálius, ‘king’); yet not until the Roman emperors adopted the title Cæsar could this word, which probably existed previously in Teut., assume the meaning ‘emperor,’ while the Romance nations adhered to the Lat. title imperator; comp. Fr. empereur. OSlov. césarǐ (in Russ. contracted Car) is derived through a G. medium (which also elucidates MidE. and OIc. keiser) from Cæsar. Thus Kaiser is the earliest Lat. word borrowed by Teut. (see Hanf). For a Kelt loan-word meaning ‘king’ see under Reich.