contronym

:  a word having two meanings that contradict one another Sometimes, just to heighten the confusion, the same word ends up with contradictory meanings. This kind of word is called a contronym. Sanction, for instance, can either signify permission to do something or a measure forbidding it to be done. Cleave can mean cut in half or stick together. A sanguine person is either hotheaded and bloodthirsty or calm and cheerful. Something that is fast is either stuck firmly or moving quickly. — Bill Bryson, The Mother Tongue, 1990 —called also antagonym, autoantonym, Janus word