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.
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