The word prohibitively 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: 28
Highest Scoring Words With Friends™ Play In The Letters prohibitively
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Seven Letter Word Alert: (29 words)
biolite, biolith, biotype, blither, brevity, brothel, hoplite, liberty, liriope, overbit, overlit, overtip, overtly, philter, philtre, pithier, pithily, pivoter, plovery, politer, potherb, poverty, preboil, probity, repilot, repivot, trilobe, tripole, vitriol