late 14c., originally in figurative sense of "unrestrained, ungoverned," from un-(1) "not" + bridled (see bridle, v.). Similar formation in Middle Dutch ongebreidelt. Literal sense of "not fitted with a bridle" (of horses) is not recorded before 1550s. The verb unbridle (see un-(2)) is attested from c.1400 in the literal sense; mid-15c. in the figurative sense.