Mammoth Uncensored Word List:
62 words ending with ss, 18 letters long

chickenheartedness comprehensibleness cotemporaneousness discomfortableness expressionlessness extemporaneousness forethoughtfulness geosynchronousness hyperconsciousness hypersensitiveness inappreciativeness inconsiderableness indestructibleness indiscriminateness interconnectedness irreconcilableness irreproachableness nondestructiveness nonfashionableness nonrepetitiousness nonrepressibleness nonrespectableness nonresponsibleness nonsupportableness nonsuppressiveness nonsusceptibleness nonsymphoniousness nonsynchronousness nontransparentness overdecorativeness overinterestedness overlasciviousness overlicentiousness overneglectfulness overprosperousness overprotectiveness overscrupulousness oversuspiciousness overtheatricalness possessionlessness presuppositionless representativeness roentgenopaqueness superconsciousness supereffectiveness superrighteousness supersagaciousness supersecretiveness supersensitiveness superstrenuousness unaccumulativeness unacknowledgedness unambidextrousness unappreciativeness unapprehensiveness unapproachableness uncompromisingness unconscionableness underconsciousness unintelligibleness unsatisfactoriness willingheartedness