in·cred·i·ble 
(ĭn-krĕdə-bəl)
(ĭn-krĕdə-bəl)adj.
1. So implausible as to elicit disbelief; unbelievable: gave an incredible explanation of the cause of the accident.
2. Astonishing, extraordinary, or extreme: dressed with incredible speed.
[Middle English, from Latin incrēdibilis : in-, not; see IN-1 + crēdibilis, believable; see CREDIBLE.]
in·cred′i·bili·ty, in·credi·ble·ness n.
in·credi·bly adv.