Mammoth Uncensored Word List:
22 words ending with m, 17 letters long

anticommercialism congregationalism constitutionalism deconstructionism denominationalism electrocardiogram electrogastrogram electroretinogram latitudinarianism overindustrialism pneumomediastinum pneumopericardium postimpressionism postmillennialism predestinarianism premillenarianism radiochromatogram reconstructionism transcendentalism ultraconservatism uniformitarianism valetudinarianism