The word tirelessness 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: 14
Highest Scoring Words With Friends™ Plays In The Letters tirelessness
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Seven Letter Word Alert: (34 words)
eeriest, enlists, entrees, entries, inserts, lenites, lessees, lessens, listens, nesters, nestles, netless, relents, relines, relists, renests, resents, resists, retiles, seitens, sessile, sestine, silents, sinless, sinters, sisters, steelie, sterile, teenier, tensile, tieless, tinsels, tresses, trienes