The word convincingness 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: 30
Highest Scoring Words With Friends™ Play In The Letters convincingness
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Seven Letter Word Alert: (29 words)
cession, ciscoes, cognise, coining, concise, conning, connive, consign, cosigns, cosines, covines, ensigns, eosinic, iciness, incises, innings, inosine, invoice, ionises, novices, sconces, sensing, siccing, sieving, sinning, veining, venison, visions, voicing