late 14c., person who has power of judging absolutely according to his own pleasure in a dispute or issue, from Old Frencharbiter, judge (13c.) and directly from Latinone who goes somewhere (as witness or judge), in classical Latin used of spectators and eye-witnesses; specifically in law, he who hears and decides a case, a judge, umpire, mediator; fromto come, go, a word of unknown etymology.

was regular in non-initial syllables (especially in Plautus). Where

occurs independently (4x in Plautus), it must be a decompounded form. [de Vaan]

The specific sense of one chosen by two disputing parties to decide the matter is from 1540s. Comparearbitrator. The earliest form of the word attested in English is the fem. nounarbitress(mid-14c.) a woman who settles disputes.Gaius Petronius Arbiter(circa 27-66 C.E.) was a friend of Nero, noted voluptuary, reputed author of the Satyricon, and an authority on matters of taste and style (elegantiae arbiter, punning on the name).