The word unassignable 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™ Plays In The Letters unassignable
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Seven Letter Word Alert: (54 words)
abasing, abseils, abulias, abusing, alanine, aliases, anginal, anginas, anneals, annuals, assuage, bagless, bangles, belugas, bigness, blueing, blunges, bungles, bunnies, bussing, ensigns, ensuing, galenas, genials, guanine, guineas, gunless, iguanas, inulase, lasagne, leaning, leasing, lensing, lesbian, ligases, nabales, nebulas, salines, sausage, seabags, sealing, sensing, sensual, siennas, signals, silanes, singles, sunnies, unagile, unalign, unbales, unbless, unseals, unsling