The Roman god Janus–the personification of a Latin word meaning “doorway”–was depicted as having two faces, each pointing in opposite directions. He was the god of doorways and gateways, beginnings and endings.
The term “Janus words” is applied to words that can mean opposites. A common example is the verb cleave, which can mean either, “to stick together” or “to cut apart”:
Gawain cleaves off the stranger’s head in one blow, but the stranger does not die.
And said, For this cause shall a man leave father and mother, and shall cleave to his wife: and they twain shall be one flesh? –Matthew 19:5, KJV.
Such words are variously known as auto-antonyms, antilogies, enantiodromes, and contranyms.
Because of the long-established term antonym as the word for “a word that is the opposite or antithesis of another,” it seems that auto-antonym is the most practical choice.
Here are three examples of auto-antonyms:
The adjective sanguine is from the Latin for “bloody.” It can be used in a literal sense: “The sanguine murders were the work of a serial killer.”
In medieval philosophy, people were believed to be governed by the “four humours”: blood, yellow bile, black bile, and phlegm.” These humors embodied certain characteristics:
blood: courageous, hopeful, amorous
yellow bile: easily angered, bad-tempered
black bile: despondent, sleepless, irritable
phlegm: calm, emotional
Someone of a “sanguine temperament,” for example, is governed by a cheerful disposition.
The auto-antonym sanguine can mean either “bloody, bloodthirsty,” or “cheerful, loving.”
The verb sanction comes from a Latin noun, sanctionem, which meant something that was so important or sacred that it was required; the law even imposed a penalty for failure to perform it. Both good and bad notions, therefore, attached to the word.
As an English verb, sanction can mean either, “to endorse or authorize,” or “to punish.” For example,
Court will sanction Prenda lawyers if they don’t appear April 2 (i.e., will punish them)
Illinois Becomes 20th State to Sanction Therapeutic Use of Cannabis (i.e., approve)
The verb dust originated with a Germanic noun that probably meant “that which rises or is blown in a cloud, like vapor, smoke, or dust. ” In modern usage, the verb dust can mean either “to remove dust particles from a surface,” or “to sprinkle dust particles on a surface.” For example,
I want you to dust the furniture before the guests arrive. (remove the dust)
The last step is to dust the cake with powdered sugar. (apply a dusting of sugar)
The use of auto-antonyms usually offers no difficulty to native speakers because the meaning is usually clear from their context. ESL speakers may have trouble with them.