the GNU version of the Collaborative International Dictionary of English
adj. Not credible; surpassing belief; too extraordinary and improbable to admit of belief; unlikely; marvelous; fabulous.
The Century Dictionary and Cyclopedia
Not credible; that cannot be credited; surpassing the possibility of belief.
Surpassing belief as to what is possible; hard to believe; unimaginable; inconceivable.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
adj. beyond belief or understanding
"He asked them to lead a fundraising drive for Haiti, praising their efforts during past disasters and saying they can tap what he calls the incredible generosity and can-do spirit of the American people."