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From letters F, L, I, R, T, A, T, I, O, U, S, N, E, S, S  I made following words

15 letter words

flirtatiousness[18]

12 letter words

frontalities[15], titaniferous[15], tristfulness[15], solitariness[12], orientalists[12], elutriations[12]

11 letter words

infiltrates[14], filtrations[14], flirtations[14], fluorinates[14], flirtatious[14], refutations[14], literations[11], orientalist[11], antistories[11], elutriation[11], neutralists[11], staurolites[11], lustrations[11]

10 letter words

antilifers[13], reflations[13], frostlines[13], satisfiers[13], frostiness[13], infiltrate[13], trifoliate[13], filtration[13], flirtation[13], stratifies[13], frontlists[13], unifoliate[13], fluorinate[13], infuriates[13], faultiness[13], sulfonates[13], sulfations[13], fruitiness[13], artfulness[13], fusionists[13], transfuses[13], refutation[13], flauntiest[13], fortuities[13], fortunates[13], stultifies[13], lanosities[10], solitaires[10], solitaries[10], seriations[10], serialists[10], assertions[10], literation[10], tonalities[10], iterations[10], natrolites[10], saintliest[10], siltations[10], tanistries[10], sterilants[10], restations[10], stationers[10], striations[10], siltstones[10], antistress[10], resistants[10], straitness[10], insulators[10], sensualist[10], sustainers[10], sultriness[10], sinistrous[10], titularies[10], neutralist[10], staurolite[10], lustration[10], situations[10], ritualists[10], resultants[10], turnstiles[10], routinists[10], tessituras[10], trustiness[10]

9 letter words

finalises[12], fissional[12], frailness[12], foresails[12], farnesols[12], salsifies[12], fossilise[12], sanserifs[12], ossifiers[12], antilifer[12], reflation[12], frailties[12], inflaters[12], forestial[12], introfies[12], notifiers[12], flatirons[12], inflators[12], frostline[12], falsities[12], finalists[12], loftiness[12], satisfier[12], filisters[12], seafronts[12], frontless[12], satisfies[12], flossiest[12], firstness[12], frostless[12], flintiest[12], floatiest[12], fetations[12], filtrates[12], flirtiest[12], frontlets[12], frontlist[12], fattiness[12], falsettos[12], fistnotes[12], frostiest[12], frauleins[12], nefarious[12], fluorines[12], fusiliers[12], furanoses[12], russifies[12], infuriate[12], flauntier[12], ultrafine[12], sulfonate[12], sulfation[12], filatures[12], unfairest[12], flaunters[12], trainfuls[12], fluorites[12], fruitions[12], sensorial[9], lionisers[9], sessional[9], solarises[9], saintlier[9], oralities[9], solitaire[9], seriation[9], orientals[9], relations[9], serotinal[9], tensorial[9], alienists[9], insolates[9], listerias[9], serialist[9], snarliest[9], sinistral[9], ostiaries[9], assertion[9], senoritas[9], sanitises[9], saltiness[9], slatiness[9], stainless[9], satirises[9], stairless[9], artsiness[9], insisters[9], assentors[9], starnoses[9], arsonists[9], iteration[9], natrolite[9], tentorial[9], siltation[9], sterilant[9], troilites[9], restation[9], stationer[9], striation[9], trotlines[9], taintless[9], totalises[9], siltstone[9], resistant[9], straitens[9], tartiness[9], slatterns[9], strontias[9], tenorists[9], tastiness[9], satirists[9], sitarists[9], arsenious[9], lousiness[9], surliness[9], ossuaries[9], retinulas[9]