The word impenetrable 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: 23
Highest Scoring Words With Friends™ Play In The Letters impenetrable
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Seven Letter Word Alert: (85 words)
ailment, aliment, ambient, arenite, balmier, beamlet, beeline, beermat, bimetal, biplane, bleater, bleeper, element, emirate, empanel, emplane, emptier, enabler, enteral, eternal, impaler, impanel, implant, inepter, latrine, lempira, lineate, maltier, manlier, marlite, matinee, mealier, meatier, meatpie, minaret, mineral, naptime, netmail, nimbler, painter, palmier, peatier, penlite, pentail, perlite, pertain, plainer, planter, plebian, praline, preemie, prelate, premeet, premelt, preteen, primate, raiment, ratline, reblame, reblent, reliant, repaint, repanel, replait, replane, replant, replate, repleat, replete, reptile, retinal, tapemen, teenier, tempera, tenable, terpene, terpine, timbale, timbrel, trainee, trample, tremble, trenail, triable, triaene