According to my Collins English Dictionary (30th anniversary edition) the primary meaning of abysmal is "immeasurable; very great; abysmal stupidity".
Then it goes on to meaning 2, which it calls informal: "extremely bad; an abysmal film".
Well, the dictionary can say what it likes, but no one, no one, uses the word abysmal informally.
As for abysmal's primary meaning, an abysmal performance is never, ever, a great one, however deep it is; and the height of the arch of a rainbow may be immeasurable but it cannot under any circumstances be described as abysmal.
Still, this is a good thing.
After all, there are plenty of things around that can be called abysmal, and it'd be a pity to overwork the word.
Thing Not To Be Today: abysmal. This word is linked to the word abyss, which comes from the Greek abussos, which means bottomless (though not in the catwalk model sense) from bussos, depth.