Words evolve over time. From the moment a word is conceptualized, it’s defined. Yet definitions change over time through every day application.
Knowing what we know now. How words are used now, alters how we react when we see those same words used in historical context.
Retardation does not mean what it means today, and how is that? Because words become redefined through application.
Of course, retardation is just one of many examples. Please, share your examples. No need for trigger warnings.