Is terribleness a word?

adj. 1. Causing great fear or alarm; dreadful: a terrible bolt of lightning; a terrible curse.

What is slang for terrible?

informal crummy, pathetic, pitiful, useless, lousy, appalling, abysmal, dire. British informal duff, chronic, poxy, rubbish, pants, a load of pants. North American vulgar slang chickenshit. brilliant.