c.1175, "stake or rod of iron used to fasten a door or gate," from O.Fr. barre, from V.L. *barra "bar, barrier," which some suggest is from Gaulish *barros "the bushy end," but OED regards this as "discredited" because it "in no way suits the sense." Bar code first recorded 1963. Behind bars "in prison" is from 1951. Meaning "bank of sand across a harbor or river mouth" is from 1586, so called because it was an obstruction to navigation.
bar (2)
"tavern," 1592, from the bars of the barrier or counter over which drinks or food were served to customers (see bar (1)). Barmaid is from 1772; bar-tender is 1836, Amer.Eng.; barfly "habitual drunkard" is from 1910.
bar (3)
"whole body of lawyers, the legal profession," 1559, a sense which derives ultimately from the railing that separated benchers from the hall in the Inns of Court. Students who had attained a certain standing were "called" to it to take part in the important exercises of the house. After c.1600, however, this was popularly assumed to mean the bar in a courtroom, which was the wooden railing marking off the area around the judge's seat, where prisoners stood for arraignment and where a barrister (q.v.) stood to plead. As the place where the business of court was done, bar in this sense had become synonymous with "court" by c.1330.
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(在線英語詞源字典)