Rather spitefully, it's the name of something particularly magical and lovely:
photo by Freestyle nl. The cupule is on the left (the other thing is a gall).
Yes, a cupule is the cup which holds an acorn:
photo by Dcrjsr
(Those are acorns from the American Northern Red Oak, Quercus rubra. Don't they have lovely Tam o'Shanter hats (or cupules, if you must be scientifically accurate)?)
Oh dear. Precision is all very well, but not at the expense of stripping away any beauty from the world, so I'm going to forget this word as soon as I can.
It's a rare example of ignorance being really rather blissful.
Word Not To Use Today: cupule. This word only arrived in English in the 1800s, which proves we can very well do without it. It comes from the Latin cūpula, a small cask, from cūpa, tub.