I thought underflow was a term used in floating point computations when you formed the difference of two large nearly equal numbers or the quotient of a very small number by a very large one and get a spurious 0.
“Gruntled” exists, but is essentially the same as “disgruntled”. They are both related to “grunt”.
huh.
Here’s a question for all you telligent tellectuals to ponder: A city often has outskirts, but does it ever have inskirts?
Nocent is another - it’s the original opposite of innocent. We now use guilty.
You don’t tend to see gormful all that often; gorm is a lovely old word for wisdom. I’ve been known to use it as a compliment - I really like using words that seem on first parse to mean the opposite. (Similarly for pulchritude, while pusillanimous and egregious don’t sound nearly as bad as they mean)
While whelmed does exist, it’s a synonym for overwhelmed.
Has there ever been an “onhand”? Offhand, I don’t know.
On the other hand…
I just saw the thread title, and thought I’d mention this song.
As others have pointed out, “canny” is a perfectly cromulent word, though it isn’t the opposite of “uncanny.”
“Couth” is also a perfectly cromulent word. I’ve heard it used many times.
My own contribution: nonplussed. Though my dictionary says that it is quickly becoming its own antonym.
When the situation warrants, I’m fond of telling people that I am exactly the appropriate amount of whelmed.
I remember once learning that flammable and inflammable have different meanings: flammable things will burn if exposed to flame, while inflammable things might combust even without coming into contact with a flame.
BUT, damned if I can find any evidence on line for this – dictionary.com provides identical definitions. It seems to make sense though. 
Not in this case. Apt is a rather common word.
“Couth” and “canny” are not joke words.
“Ruthless” is the negation of “rue” which is not all that rare.
Ditto “inept” - “apt” and “heedless” - “heed”.