The word thankfulness 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: 25
Highest Scoring Words With Friends™ Play In The Letters thankfulness
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Seven Letter Word Alert: (35 words)
anklets, annulet, auklets, eluants, enfants, falsest, fankles, fastens, fatness, flashes, flaunts, fluents, flushes, funnels, funnest, hastens, hateful, hatfuls, hatless, hustles, lunates, lushest, salutes, sensual, sleuths, sulfate, sultans, sunhats, suntans, taluses, tankful, tunnels, unleash, unseals, unseats