late 14c., reducen, "bring back" (to a place or state, a sense now obsolete), also "to diminish" (something), from Old French reducer (14c.), from Latin reducere "lead back, bring back," figuratively "restore, replace," from re- "back" (see re-) + ducere "bring, lead" (from PIE root *deuk- "to lead").
In Middle English largely with positive senses, including "bring back to virtue, restore to God; bring back to health." The specific meaning "bring to an inferior condition" is by 1570s; that of "bring to a lower rank" is by 1640s (military reduce to ranks is from 1802); that of "subdue by force of arms" is from 1610s. The sense of "to lower, diminish, lessen" is from 1787. Related: Reduced; reducing.