Nuts and Bolts - working words.
There are some words that nearly everyone understands (NO!) and some words that very few people understand (FETTLING*).
And there are some words NO ONE AT ALL understands. (And I don't just mean nonsense words like MIBBLE and TOVE.).
For instance, if I told you my mug was quite empty, how full would it be? No one can be sure.
Or, as another example, have you ever had that awkward conversation when you've been ill and someone asks you if you're better, and you end up saying, I'm better, but I'm not better?
Words like this, that mean themselves and something pretty much the opposite as well, are called contranyms.
Can you think of another one?
A CLUE: think of prunes!
*Stuff used to line the hearth of puddling furnaces, apparently.