late 14c., 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.