bench-warmer (n.)

Middle English bench, from 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). The sporting sense "reserve of players" (in baseball, North American football, etc.) is by 1909, from a literal sense in reference to 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.