I looked up "terribad", since it seemed an unlikely typo. The luminaries at Urban Dictionary told me it was "[t]he act of being terrible and bad at the same time"; I can't for the life of me see how something could be terrible and not also be bad.
> I can't for the life of me see how something could be terrible and not also be bad.
Terrible doesn't necessarily imply bad, it can also imply fear or awe, in which case it's perfectly possible to be terrible and not bad. It's similar in that way to 'awful', which these days is used almost exclusively for something that is bad, but occasionally you'll still find the old usage (awe-filling) appearing.
On the other hand, 'terribad' is the kind of word that fills me with dread (combining latin and old english roots like that? The outrage!). I know language is arrived at by consensus but even if there were consensus on 'terribad', I'd not be joining it.
Since you asked, it's possible that terrible things can be both "good" in the sense of 'not evil' ("he's a terrific guy") and "good" in the sense of 'well done' ("Oz, the Great and Terrible").
