1. not easy to believe

Similar word(s): implausibly, improbably, incredibly

2. in an unbelievable manner

- he was unbelievably angry

Sentences with unbelievably as an adverb:

- He gestured unbelievably. Everyone knew he was faking being sick.

- His face turned unbelievably blue. We realized he was actually choking.

- Unbelievably, Rudy saved the man's life but got sued for breaking two of his ribs.