As if English isn’t one of the hardest languages to learn, Janus words are just as frustrating.  Janus words are words that have developed contradictory meanings—like 'fast', which can refer both to moving very quickly and to staying put. 

They're also known as contronyms, antagonyms, auto-antonyms (shudder), and enantiodromia (fun!). The idea came about after the two-faced Roman god Janus, who presided over doorways, gates, and transitions (and so gave his name to January, which faces back into the old year and forward to the new). Janus Words can be confusing, but I'll give a few examples.

Cleave is often cited as the go-to contronym: which may mean both "cling to" and "rend asunder." The former sense can be seen in this Genesis verse about the first man ever to exist being too clingy with a female he just met:

“Therefore a man leaves his father and his mother and cleaves to his wife, and they become one flesh.”

Whereas you see the opposite meaning in Shakespeare’s writing “He would drown the stage with tears, and cleave the general ear with horrid speech” — which is Hamlet being rather too hard on himself. However, it is not the only one out there, and most auto-antonyms have some sort of logic behind them.

The inverse also occurs: a word with a broad meaning develops a number of more specialised meanings in parallel, but in such a way that two opposing and later meanings result. One of these words is sanction. It alluded to an oath when translated into English. It later developed to refer to two contradicting connotations that allude to approbation and economic disapproval—both of which could compel a person or a country to act better. 

One type of Janus word is especially perplexing: those with opposite meanings on either side of the Atlantic. In the United Kingdom, "moot" denotes" something which can be debated; arguable," whereas in the UnitedStates, it means "not worthy of discussion."

Some of these oddities arise from mere coincidence. The two meanings of "cleave" are essentially different terms that are written similarly in modern English. (This is why the past participle "cleft" appears in the "cutting" form but not in the other.)

On the other hand, others are the result of drift. Look up any common word in a historical dictionary, such as the Oxford English Dictionary, and you'll find that the oldest meaning is often shockingly different from the current one. ("Silly," for example, isa distant cousin of "soul" and previously meant "holy.")The Janus words appear when a new meaning diverges while the old one persists.So the first meaning of "fast" is "stuck in place," but as it evolved, it became an adverb meaning to accomplish something with vigour. The"moving intensely"—that is, quickly—was most likely derived from the"intensity" notion.

Finally, some Janus terms are the result of plain confusion. The term "inflammable" has been used to mean both "able to catch fire" and "unable to catch fire."That first meaning is the more traditional one, sharing a root with"inflame." However, in- is a common negative prefix, which leads to the second interpretation. So it is with "comprise," which may have gotten its more contemporary meaning (the elements that make up the whole) from confusion with its relative, "compose."

There is so much more to it through the process of gradual change while the old meaning endured. For now, I shall stop rambling!

I’d say it is tempting to think that such self-contained opposites cannot and should not coexist. But who really cares? Unless you’re a Wikipedia editor or a huge fan of the 16th century, all this is really just a tolerable state of affairs that everyone brushes aside. The ever prevalent English language will be here to stay, for along time.