The word doubtlessly 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 doubtlessly
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Seven Letter Word Alert: (34 words)
bedusts, bestuds, bloused, blouses, boldest, boluses, budless, budlets, bullets, bustled, bustles, dotless, doubles, doublet, dullest, lobules, lotuses, loudest, outbled, outsell, outyell, sellout, soluble, solubly, solutes, sublets, subslot, systole, tousled, tousles, toyless, tubless, tussled, tyloses