/ ˌʌnbɪˈliːvəbl; NAmE / adjective
1.
used to emphasize how good, bad or extreme sth is
SYN incredible :
We had an unbelievable (= very good) time in Paris.
Conditions in the prison camp were unbelievable (= very bad) .
The cold was unbelievable (= it was extremely cold) .
It's unbelievable that (= very shocking) they have permitted this trial to go ahead.
2.
very difficult to believe and unlikely to be true
SYN incredible :
I found the whole story bizarre, not to say unbelievable.
OPP believable for sense 2
► un·believ·ably adverb :
unbelievably bad / good
Unbelievably it actually works.