(literally) Not to be believed. / (figuratively) Amazing, incredible; so surprising it is almost unable to believe. / Implausible or improbable.

The state or quality of not being believable.

Not believed; disbelieved

One who does not believe, particularly in a deity (used by believers to describe people who do not believe in the same deity or deities as themselves).

(manner) In a manner that one does not or cannot believe. / (degree) To an extent not to be believed. / (evaluative) Contrary to expectations, amazingly.

plural of unbeliever

That does not believe; incredulous, skeptical.

The state or quality of being unbelievable.

(transitive, intransitive) To lose, abandon, or relinquish belief in; stop believing.

Third-person singular simple present indicative form of unbelieve