kaiser (n.)

"an emperor, a ruler, a dictator," late 14c., cesar, from Cæsar, originally a surname of the Julian gens in Rome, elevated to a title after Caius Julius Caesar (100 B.C.E.-44 B.C.E.) became dictator; it was used as a title of emperors down to Hadrian (138 C.E.). The name is of uncertain origin; Pliny derives it from caesaries "head of hair," because the future dictator was born with a full one; Century Dictionary suggests Latin caesius "bluish-gray" (of the eyes), also used as a proper name. Also compare caesarian.

Old English had casere, which would have yielded modern *coser, but it was replaced in Middle English by keiser (c. 1200), from Norse or Low German, and later by the French or Latin form of the name. Cæsar also is the root of German Kaiser and Russian tsar (see czar). He competes as progenitor of words for "king" with Charlemagne (Latin Carolus), as in Lithuanian karalius, Polish krol.

The use in reference to "temporal power as the object of obedience" (contrasted with God) is from Matthew xxii.21. Caesar's wife (1570s) as the figure of a person who should be above suspicion is from Plutarch. In U.S. slang c. 1900, a sheriff was Great Seizer.

early 13c., from Old French empereor "emperor, leader, ruler" (11c.; accusative; nominative emperere; Modern French empereur), from Latin imperatorem (nominative imperator) "commander, emperor," from past participle stem of imperare "to command" (see empire).

Originally a title conferred by vote of the Roman army on a successful general, later by the Senate on Julius and Augustus Caesar and adopted by their successors except Tiberius and Claudius. In the Middle Ages, applied to rulers of China, Japan, etc.; non-historical European application in English had been only to the Holy Roman Emperors (who in German documents are called kaiser), from late 13c., until in 1804 Napoleon took the title "Emperor of the French."