Ironic

People think it means:
Any kind of amusing coincidence.

Actually means:
An outcome that is the opposite of what you'd expect.

So, if a porn star moved to Virgin, Utah, that would be ironic. If the same porn star bought a house in Boner Knob, Montana that would not be ironic.

Should you care?
We realize this is a technical point. But, it's almost worth taking a stand because the word has been abused to the point that it can mean anything.

Pristine

People think it means:
"Spotless" or "as good as new."

Actually means:
"Ancient, primeval; in a state virtually unchanged from the original."

It's therefore perfectly possible to have a pristine mountain of fossilized brontosaurus shit, but if you were to buff that mountain to a lustrous shine, it would no longer be pristine.

Should you care?
The meanings are close enough that correcting somebody sounds like grammar Nazi hair-splitting. That's a shame, because there were lots of words that mean "clean" but none that have the exact same meaning as "pristine."

If you use pristine correctly yourself, you probably won't land yourself in too much trouble, unless someone buys your "pristine" house on eBay without realizing that it's an authentic 14th century dung hovel complete with never-been-used plague rats.

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Nonplussed

People think it means:
Unperturbed, not worried.

Bemused

People think it means:
Mildly amused.

Actually means:
Bewildered or confused.

If you were to say "I was bemused by your dead baby joke," you wouldn't be saying the joke was funny. You'd be saying that you completely failed to understand it. You were following the story up to and including the bit about the trowel, but you'd lost the thread way before the Ku Klux masturbation climax.

Should you care?
It's hard to blame people for getting this one wrong, the word just sounds like it means, "sort of amused." We blame the people who originally invented the word. You should probably let the new meaning take over unless, you know, you're a dick.

Enormity

People think it means:
Enormous.

Actually means:
Outrageous or heinous on a grand scale.

War crimes are enormities. Extra-big bouncy castles are not.

Should you care?
This is one of those words you really don't need to be using anyway, unless you're giving a speech at the U.N. Just remember that if you say to your girl, "I hope you're prepared for the enormity of my dick," you're implying that your penis is responsible for several acts of evil on the scale of ethnic genocide. This may or may not turn her on, depending on the girl.

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Plethora

People think it means:
A lot of something.

Actually means:
Too much of something, an over-abundance.

It's the difference between:

"Dude, I am jonesing to go snort a plethora of medicinal-grade barbiturates right now."

And ...

"Dude, I just snorted a plethora of medicinal-grade barbiturates, and now there are hundreds of terrifying arachnids crawling out of my penis. They all have human lips."

Should you care?
As with "enormity," you're courting a certain amount of dickery by using "plethora" at all; most of the time, you can get the same point across by saying "a big ol' shitload." However, the original meaning of over-abundance is worth hanging onto, because it seems as if there's no direct replacement other than "too many big ol'shitloads," which doesn't have quite the same degree of pith.

Deceptively

People think it means:
Nobody is sure.

Actually means:
Nobody is sure.

Specifically, we're talking about when the word is used with some other adjective. Like if somebody says, "The turd pool is deceptively shallow," does that mean it's deeper than it appears, or not as deep?

If you're not sure, don't feel bad. The American Heritage Dictionary asked their word experts and they said they had no fucking idea, either. So ... nobody knows.

Should you care?
So, if you say (to a lady, perhaps), "I possess a deceptively large set of balls," you could mean that your modest bulge belies the real heft of your testicles, which are actually so pendulous that you're forced to strap them to your legs. However, you could also mean that you have tiny love eggs, and that your ball-shaped jean protrusions are actually caused by the hideous malformation of your wang. This is obviously something you want to avoid.

If ever there was a case to be made for clarity of language, this is it. If you use it at all, make sure the context makes the meaning totally clear. "My balls are deceptively large," you could say, "because I have just inflated my genitalia with a bicycle pump."

What this also means is that technically the usage is never wrong ... or right. If you're the type who just likes to correct people to be a dick, well, this one is a gold mine.

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For more ridiculous things you didn't know about your native language, check out 8 Everyday Words With X-Rated Origins and The 10 Coolest Foreign Words the English Language Needs.