Is plump good? Well, it is if you're a wallet or a goose or a cushion.
But is skinny better if you're a person?
Well, I'm not certain about that, but plump is by far the more endearing word.
Plump, plump, plump...
Luckily you can join a plump gathering and still be skinny, because a plump is an old word for a group of people or animals (or indeed, almost anything else).
Even nowadays, this meaning still has some resonance. I'm sure you've seen a plump of lunching businessmen, for instance.
If you yourself are well-cushioned then you will be able to plump yourself down on a chair without discomfort. Is this a coincidence?
History doesn't tell us, but I doubt it.
Thing To Be Today: plump. This word started off in the 1400s meaning dull or rude, but by the 1500s it had its modern meaning. It's probably something to do with the Dutch word plomp, which means dull or blunt.
The dropping-down word is probably imitative.
No one knows from where the group-of-things word comes.