The word irresistibility 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: 21
Highest Scoring Words With Friends™ Play In The Letters irresistibility
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Seven Letter Word Alert: (31 words)
bitsets, bitsier, bitters, bittier, blister, bristle, bristly, brittle, elitist, ileitis, liberty, litters, lyrists, relists, restirs, retilts, ribless, riblets, risible, siltier, sitters, slitter, stilter, stylers, stylets, stylier, stylise, stylist, testily, tilters, tryster