So implausible as to elicit disbelief: gave an incredible explanation of the cause of the accident.
Astonishing: dressed with incredible speed.
[Middle English, from Latin incrēdibilis : in-, not; see in-1 + crēdibilis, believable; see credible.] in·cred'i·bil'i·ty, in·cred'i·ble·ness n., in·cred'i·bly adv.
1412, from L. incredibilis "that cannot be believed," from in- "not" + credibilis "worthy of belief" (see credit). Incredulity "disbelieving frame of mind" first attested 1430; incredulous "unbelieving" is from 1579.