"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.