mid-14c., "condition of being in active operation; practice for the sake of training," from Old French exercice (13c.) "exercise, execution of power; physical or spiritual exercise," from Latin exercitium "training, exercise," from exercitare, frequentative of exercere "keep busy, drive on," literally "remove restraint," from ex- "off" (see ex-) + arcere "keep away, prevent, enclose," from PIE *ark- "to hold, contain, guard" (see arcane).
Original sense may have been driving farm animals to the field to plow. Meaning "physical activity" first recorded in English late 14c.; in reference to written schoolwork from early 17c. The ending was abstracted for formations such as dancercise (1967); jazzercise (1977); and boxercise (1985).