Old English benc "long seat," especially one without a back, from Proto-Germanic *bankon(source also of Old Frisian bank "bench," Old Norse bekkr, Danish bænk, Middle Dutch banc, Old High German banch). The group is cognate with bank (n.2) "natural earthen incline beside a body of water," and perhaps the original notion is "man-made earthwork used as a seat."
Used from late 14c. of a merchant's table. From c. 1300 in reference to the seat where judges sat in court, hence, by metonymy, "judges collectively, office of a judge." Hence also bencher "senior member of an inn of court" (1580s). Sporting sense "reserve of players" (in baseball, North American football, etc.) is by 1909, from literal sense of place where players sit when not in action (attested by 1889). A bench-warrant (1690s) is one issued by a judge, as opposed to one issued by an ordinary justice or magistrate.