degeneration theory/the concept of social degeneration was not invented and not even popularized by the nazis (and not even by max nordau as that post going around claims), and I do think it's misleading to focus on this particular use to justify why calling people degenerate is wrong. it rests on another instance of nazi exceptionalism, as if they represented the prime example of all those ideologies.
the concept of degeneration itself is indebted to evolution theories and discourses of race and origin in the eighteenth century - the construction of a racial taxonomy where each person's individual weakening meant a weakening of their entire race, and of entire populations being, biologically, weaker or predisposed to weakness. it is also tied to progressive urbanization which generated new discourses towards "urban crowds", notably along medical and hygienic lines. these ideas mostly found footing initially in 18th century france (through the works of lamarck and buffon for example) and germany. medical developments at the time - identification of the cause for syphillis, for example, or pasteur's work on vaccines, or works on fetal alcohol snydrome - are not separable from this idea of regenerating the national stock, especially in times of national crises as was experiencing france (military defeat to prussia, declining birth rates, communist activity) at the time.
even regarding nordau, the context of the french third republic is relevant to the rise of modern zionism; theodore herzl was working as a correspondent in paris during the dreyfus affair, and was receptive both to european antisemitism and to the possibilities exemplified by the european scramble for africa regarding resource extraction and settler colonialism. but nordau didn't come up with these theories himself, and many writers, politicians, or intellectuals that now enjoy a legacy of being enlightened, progressive, modern thinkers (emile zola, arthur conan doyle, jean-martin charcot, etc.) espoused similar ideas, and understood the world in those terms, though not all of them used the word degeneration specifically (not least of all due to privileging national-made concepts in a context of inter-european conflict).
degeneration theory found its first mainstream articulation in especially scientific and medical terms with benedict morel (born almost 50 years before morau), whose ideas proved itself very relevant in the framework of the french third republic regarding social hygiene policies. this is relevant because pinning eugenistic and exterminatory politics and theories onto national socialism serves to exempt liberal european democracies in which those ideas arose and cristallized. morel was a psychiatrist who developed his theories in asylums; degeneration theory cannot be separated from the rise in psychiatric power and the expanding nosology of psychiatric disorders. the mental deficiency act of 1913 in the united kingdom, the understanding of mental illness as biologically mediated and innate, alcohol prohibition in the united states, the concept of OCD itself, or modern criminology are no less linked and indebted to degeneration theory (and its associated concepts) than nazi policies. they are in fact a continuum!
as always, this is less a case of instructing people not to use the word degenerate specifically (though I don't think anyone should!) and more about familiarizing yourself with the rhetoric around a weakening of intellectual faculties, of modernity and modern technologies leading to an irrepressible decline, of an absence of willpower leading to swathes of population becoming lazy, fat, disabled, etc., a framework that expands much beyond the category of person that uses the word degenerate!
Faces of Degeneration: A European Disorder, c.1848-c.1918, by Daniel Pick
Crime, Madness and Politics in Modern France: The Medical Concept of National Decline, by Robert A. Nye
gracieabrams
blakekathryn
