1858 in reference to the emperors of Austria and (after 1870) Germany, from German Kaiser, Bavarian and Austrian spelling variant of of Middle High German keisar, from Old High German keisar "emperor," an early borrowing of Latin cognomen Caesar.
The Germanic peoples seem to have called all Roman emperors "caesar" (compare Old English casere, Old Norse keisari "an emperor"). The word also entered Germanic via Gothic, perhaps from Greek. According to Kluge, one of the earliest Latin loan word in Germanic. The Old English word fell from use after Middle English.