The word semitrailer 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: 15
Highest Scoring Words With Friends™ Plays In The Letters semitrailer
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Seven Letter Word Alert: (77 words)
aeriest, airiest, airtime, alerter, armlets, armrest, artsier, earlier, elaters, elitism, emirate, irisate, itemise, limiter, mailers, maltier, marlite, marries, mealier, meatier, melters, metrise, militar, mistier, mistral, miterer, realers, realest, realise, realism, realist, realter, reamers, rearise, reemits, relater, relates, reliers, remails, remates, remelts, resmelt, retails, reteams, retears, retiles, retimer, retimes, retires, retrial, retries, retrims, saltier, seamier, seriate, seriema, serrate, siltier, similar, siriema, slatier, slimier, smarter, smartie, smearer, smelter, smilier, stealer, steamer, sterile, tarries, tearers, tearier, trailer, tramels, trimers, trireme