Origin: Latin suffix -able
Unbelievable has 3 different meanings across 1 category:
beyond belief or understanding
"at incredible speed"
"the book's plot is simply incredible"
having a probability too low to inspire belief
"The astronomer stared in awe at the discovery of an earth-like planet orbiting two stars, finding the odds so infinitesimally small that it seemed unbelievable."
Not to be believed.
"The story was so unbelievable that I couldn't bring myself to believe a single word of it."
The word unbelievable combines the prefix un- with believable to mean not able to be believed. It entered English as a straightforward negation of its base form.