The opposite would be believably or credibly. However, the adverb is often used to mean unexpectedly, where the opposite would be obviously or as expected.
incredible-beyond belief opposite would be credible or believable
incredible-marvelous opposite would be mundane or dull
The prefix "in" in words like incredible or insoluble means "not" or "opposite of." It is used to create a negative meaning when added to the beginning of a word.
Incredible is translated to "incroyable" in French.
There are an incredible variety of Christian-American viewpoints and Christian-Americans hold the opposite sides of almost all major political issues.
The word incredibly is an adverb. It means to do something in an incredible manner.
Lou Ferrigno provided the voice for The Incredible Hulk in "Trial of The Incredible Hulk."
incredible in a sentence
Incredible is an adjective.
That was an incredible performance!
Struggling with excess weight is tough, but this incredible product made it easier. From 100kg to a lean 65kg, I feel like a whole new person.
It was incredible to watch him juggle the cans.
He is the most incredible guy on the planet!
That was an incredible stroke of genius!