Mammoth Uncensored Word List:
48 - 18 letter words starting with u

ultracentrifugally ultraconscientious ultraconservatisms ultraconservatives ultramicrobacteria ultramicroscopical ultranationalistic ultrarevolutionary ultrasophisticated unacclimatizations unaccountabilities unaccumulativeness unacknowledgedness unambidextrousness unappreciativeness unapprehensiveness unapproachableness unassailablenesses unattractivenesses unbureaucratically unchangeablenesses uncharitablenesses uncompartmentalize uncompromisingness unconscionableness unconstitutionally unconvincingnesses undecicentillionth underconsciousness undersocialisation underspecification undiscriminatingly unenthusiastically ungrammaticalities uniformitarianisms unintelligibleness unpredictabilities unprinciplednesses unprofitablenesses unreasonablenesses unresponsivenesses unrestrainednesses unsatisfactoriness unscrupulousnesses unseasonablenesses unsexagintillionth unsubstantialities untrigintillionths