I only use plethora to sound pretentious. Otherwise I’d never touch it. Why I don’t know. It is kind of a strange word. It was Greek and then popped into medical Latin about 500 years ago meaning excess fluid. You sprain your ankle and it swells up like a balloon with plethora, or plethorae or plethoram depending on the case. There are four different endings for the plural in case you manage to sprain both ankles. It must have been a relief that it remained in Latin. Or would have, had not some wag turned it into a English metaphor for excess anythings about three hundred years ago and it has not shifted meaning in all the time since. It has probably always bothered some people because it still sounds more like a medical condition than a group noun. I probably use its antonym dearth more, because apparently I don’t think it’s as pretentious as plethora. They’re not the least bit related. Plethora was plucked by an intellectual from the Latin, while dearth came up the hard way, from the West Germanic, like most of English. Dearth in medieval times–derthe–had connotations of a bare cupboard, of famine. It was a scary, ugly word, and with the vagaries of food supply in that era was probably more commonly used than we could ever comprehend in our own obese times. But go back deep into the Dark Ages, in the Old Saxon from which much of our English sprung, and diurtha meant love, glory, even splendor. It was an exultant word. A thrilling word. From splendor to famine in a few centuries. That’s a long sad tale of semantic change. Now dearth simply means not enough, and will soon be forgotten altogether, as have nearly all words in all languages, eventually. Linguists educatedly guess that 80% of all languages spoken in history have disappeared, perhaps 30,000 tongues, though how they came up with that guesstimate I’ve no idea. But thirty thousand languages is a lot of words, millions of words. Some get passed on and transmogrified, like dearth. Some get dug up and repurposed, like plethora. Most disappear forever, or darisam, as a Sumerian would have said, though we only know that because it was etched in cuneiform on a mud tablet and baked in an oven and forgotten in the ruins of a long dead city, as no one has spoken Sumerian in two thousand years.
Brick's Science 