Etymology
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believable (adj.)

"credible, capable of being believed," late 14c., from believe + -able. Related: Believably.

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believability (n.)

"credibility, capability of being believed," 1865; see believable + -ity.

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unbelievable (adj.)

1540s, from un- (1) "not" + believable. Related: Unbelievably.

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credible (adj.)

"believable, worthy of belief, capable of being believed, involving no impossibility; of known or obvious veracity or competence," late 14c., from Latin credibilis "worthy to be believed," from credere "to believe" (see credo). Related: Credibly.

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