Interestingly, "plethora" once meant "an over-abundance of bodily fluids" so if you heard your doctor say this back in the 1700s, it meant they were about to stick a bunch of leeches on you.
People think it means:
Nobody is sure.
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.
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.