In a way that is so extreme or shocking that it's hard to believe; incredibly or amazingly.
Combination of 'un-' (not) + 'believe' (from Latin 'credere' meaning to trust) + '-ably' (adverb suffix). The word emerged in the 1600s-1700s as a way to express incredulity.
Teenagers say 'unbelievably good' but their parents say the same event is 'unbelievably bad'—this word doesn't describe the fact itself but the speaker's emotional reaction, making it revealing about what shocks different people.
Complete word intelligence in one call. Free tier — 50 lookups/day.