This awful post is an egregious stem-winder

And it will leave you literally nonplussed.

Certain words carry opposite meanings. They're known as autoantonyms, or, sometimes, contronyms or Janus words (after the Roman god).

Life as we know it won't end if you use these words -- context counts -- but if you're a stickler for clarity it's worth learning to recognize them. Anything that requires a reader to stop and do a computation will slow down the transfer of your message.

Novelist Brad Leithauser offers a sampling:

I don’t know how many auto-antonyms English offers, but the list includes “cleave” (unify or sever—the butcher’s wife cleaves to the butcher, who cleaves the cow’s carcass), “overlook” (oversee or fail to notice), “let” (allow or, as in the legal phrase “let or hindrance,” obstruct), “enjoin” (encourage or prohibit), and “sanction,” as in any sanctioned imports are either approved goods or contraband. (There’s a special appealing subclass of auto-antonyms that exists only when spoken, as in raze/raise a building or—if muddily enunciated—prescribed/proscribed drugs.

 They sometimes result from our misuse. Leithauser:

I suspect that “inflammable” will soon go up in smoke. There appears to be growing confusion as to whether the prefix “in” serves as intensifier (highly flammable) or negator (fire resistant). We’re probably only one big lawsuit away from the word’s near-extinction. Picture the poignant plaintiff, about to receive a multimillion-dollar settlement, explaining in broken English that he bought his daughter a blouse made of an inflammable fabric because he wanted to protect her. Picture the clothing manufacturers racing to alter their labeling.

Here from Wikipedia is a another sampling:

• "All but" can mean "except for" or "almost entirely".
• "Apparent" can mean "obvious" or "seeming, but in fact not."
• "Awful" can mean "worthy of awe" or "very bad."
• "Besides" means "other than; except for; instead of", but can also mean "in addition (to)."
• "Egregious" can mean "outstandingly bad" or in archaic writing "remarkably good."
• "Enjoin" can mean "command" and "forbid."
• "Original" can mean "first" as in "the original painting" or something completely new: "an original work"
• "Out" can mean "available" as in "the latest model is out" or "unavailable" as in "Sorry, we're out."
• "To overlook" can mean "to inspect" or "to fail to notice."

It's a funny language.