The word incredulity is playable in Words With Friends™, no blanks required. Because it is longer than 7 letters, you would have to play off an existing word or do it in several moves.
Words With Friends™ Letter Score: 20
Highest Scoring Words With Friends™ Play In The Letters incredulity
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Seven Letter Word Alert: (30 words)
century, citrine, crudely, cruelty, cutlery, cutline, diluent, diluter, dirtily, ductile, ductily, incited, inciter, include, inditer, inducer, inertly, intrude, lintier, nitride, nuclide, tendril, tindery, tiredly, trundle, uncited, untiled, untired, untried, uridine