Auto-antonym

An auto-antonym or autantonym, also called a contronym, contranym or Janus word, is a word with multiple meanings (senses) of which one is the reverse of another. For example, the word cleave can mean "to cut apart" or "to bind together". This phenomenon is called enantiosemy, enantionymy (enantio- means "opposite"), antilogy or autantonymy. An enantiosemic term is necessarily polysemic.

Nomenclature
The terms "autantonym" and "contronym" were coined by Joseph Twadell Shipley in 1960 and Jack Herring in 1962, respectively. An auto-antonym is alternatively called an antagonym, Janus word (after the Roman god Janus, who is usually depicted with two faces), enantiodrome, enantionym, self-antonym, antilogy, or addad (Arabic, singular didd).

Linguistic mechanisms
Some pairs of contronyms are true homographs, i.e., distinct words with different etymology which happen to have the same form. For instance cleave "separate" is from Old English clēofan, while cleave "adhere" is from Old English clifian, which was pronounced differently. The King James Bible often uses "let" in the sense of "forbid", a meaning which is now uncommon, and which is derived from the Old English verb lettan 'hinder, delay, impede, oppress', as opposed to the meaning "allow", which is derived from the Old English verb lǣtan 'leave, allow, let on lease (etc.)'. The alternative meaning of "let" can still be found in the legal phrase "without let or hindrance" and in ball games such as tennis, squash, table tennis, and racquetball.

Other contronyms are a form of polysemy, but where a single word acquires different and ultimately opposite definitions. For example, sanction—"permit" or "penalize"; bolt (originally from crossbows)—"leave quickly" or "fix/immobilize"; fast—"moving rapidly" or "unmoving". Some English examples result from nouns being verbed in the patterns of "add &lt;noun> to" and "remove &lt;noun> from"; e.g. dust, seed, stone. Denotations and connotations can drift or branch over centuries. An apocryphal story relates how Charles II (or sometimes Queen Anne) described St Paul's Cathedral (using contemporaneous English) as "awful, pompous, and artificial," with the meaning (rendered in modern English) of "awe-inspiring, majestic, and ingeniously designed". Negative words such as bad and sick sometimes acquire ironic senses referring to traits that are impressive and admired, if not necessarily positive (that outfit is bad as hell; lyrics full of sick burns).

Some contronyms result from differences in varieties of English. For example, to table a bill means "to put it up for debate" in British English, while it means "to remove it from debate" in American English (where British English would have "shelve", which in this sense has an identical meaning in American English). To barrack for anyone in Australian English is to loudly demonstrate your support, while it expresses disapproval and contempt in British English.

Some words contain simultaneous opposing or competing meanings in the same context, rather than alternative meanings in different contexts; examples include blend words such as coopetition (meaning a murky blend of cooperation and competition), frenemy (meaning a murky blend of friend and enemy), glocalization, etc. These are not usually classed as contronyms, but they share the theme of containing opposing meanings.

Auto-antonyms exist in many languages, as the following examples show.

In Latin, sacer has the double meaning "sacred, holy" and "accursed, infamous". Greek δημιουργός gave Latin its demiurgus, from which English got its demiurge, which can refer either to God as the creator or to the devil, depending on philosophical context.

In many languages, a word stem associated with a single event may treat the action of that event as unitary, so it can refer to any of the doings or persons on either side of the transaction, that is, to the action of either the subject or the object, or to either the person who does something or the person to whom (or for whom) it is done. Other cues nail down the aspects of subject versus object. Thus there is a simple logic involved, despite that discussions of such words sometimes fixate on a superficial appearance of illogic (that is, "how can one word mean both?!") such as in words for borrow and lend; see below.

English

 * Cleave can mean "to cling" or "to split apart".
 * Clip can mean "attach" or "cut off".
 * Dust can mean "to remove dust (cleaning a house)" or "to add dust" (e.g. to dust a cake with powdered sugar).
 * Fast can mean "without moving; fixed in place", (holding fast, also as in "steadfast"), or "moving quickly".
 * Let can mean "allow" or "prevent" (A British Passport contains a request that the bearer be allowed to "pass without let or hindrance").
 * Left can mean "remain" or "leave" ("He left the room" or "He was the last one left in the room").
 * Off can mean "activated", "beginning to make a noise" (e.g. "The alarm went off") or "deactivated" / "ceasing operation" (e.g. "The alarm turned off by itself").
 * Overlook can mean to miss seeing something, or a place to see something from above.
 * Oversight can mean "accidental omission or error", or "close scrutiny and control".
 * Peruse can mean to "consider with attention and in detail" or "look over or through in a casual or cursory manner".
 * Ravel can mean "to separate" (e.g. threads in cloth) or "entangle".
 * Sanction can mean "approve" or "penalize".
 * Table can mean "to discuss a topic at a meeting" or "to postpone discussion of a topic".

Other languages

 * Nouns
 * The Korean noun 앞(ap) may mean either "future" or "past" (distinguished by context).
 * Verbs
 * The German verb ausleihen, the Polish verb pożyczyć, the Russian verb одолжить and the Finnish verb lainata can mean either "to lend" or "to borrow", with case, pronouns, and mention of persons making the sense clear. The verb stem conveys that "a lending-and-borrowing event is occurring", and the other cues convey who is lending to whom. This makes sense because anytime lending is occurring, borrowing is simultaneously occurring; one cannot happen without the other.
 * The Romanian verb a închiria means "to rent" (as the lessee does) as well as "to let" (as the lessor does).
 * The Swahili verb kutoa means both "to remove" and "to add".
 * In his Limited Views: Essays on Ideas and Letters, Qian Zhongshu gave some examples of Chinese auto-antonyms, like "廢" meaning both "to abolish" and "to establish". He named this kind of phenomenon "reverse symbolism"(反象以徵).
 * The Persian verb چیدن means both " to pluck" and "to arrange" (i.e. by putting objects down).
 * Adverbs
 * कल and کل (kal ) may mean either "yesterday" or "tomorrow" (disambiguated by the verb in the sentence).
 * ar ball can mean both "a while ago" and "in a little bit/later on"
 * Agent nouns
 * The Italian and French cognates, ospite and hôte, respectively, also can mean "host" or "guest". Both words derive from the Latin hospes, which also carries both meanings.
 * Adjectives

Seeming auto-antonyms can occur from translation. In Hawaiian, for example, aloha is translated both as "hello" and as "goodbye", but the essential meaning of the word is "love", whether used as a greeting or farewell. The Italian greeting ciao is translated as "hello" or "goodbye" depending on the context; however, the original meaning was "(I'm your) slave".