A noun with a complex history: see LSJ s.v. Its initial meaning, in Homer, is 'he who stands beside', meaning specifically the warrior who stands in his chariot next to the charioteer. Other military uses are comparable. However, the totally different 'apostate/transgressor' sense is found as early as Aeschylus; and of course the Apostate par excellence is the emperor Julian (Flavius Claudius Iulianus, reigned 361-3 CE), who renounced Christianity for paganism: see iota 437.