c.1374, originally in fig. sense of "unrestrained, ungoverned," from
un- (1) "not" +
bridled (see
bridle). Cf. M.Du.
ongebreidelt. Lit. sense of "not fitted with a bridle" (of horses) is not recorded before 1553. The verb
unbridle is attested from c.1400 in the lit. sense; c.1440 in the fig. sense.