The word imperceptibility cannot be made in Words With Friends™. It is 16 letters long, too long to fit on a WordsWithFriends board (15 letters max).
Highest Scoring Words With Friends™ Plays In The Letters imperceptibility
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Seven Letter Word Alert: (47 words)
betitle, bittier, brittle, climber, clipper, clitter, crimple, cripple, emitter, empiric, emptier, emptily, epicyte, erectly, imperil, impiety, liberty, limiter, lippier, pelitic, percept, perlite, pettier, pettily, picrite, pipette, precept, preempt, prelect, premelt, pretype, pyritic, receipt, reclimb, reptile, retitle, ripplet, tempter, termite, timbrel, tippier, tippler, titmice, tremble, trembly, triplet, yperite