John Dewey Will Rewrite Your Dictionary
I'd add a pragmatist angle to this. In a pragmatist philosophy of language, words are tools. We encounter problems in going about our business (for example, various harms that appear to be race-related), then come up with concepts to help us guide action in response to those problems, and then find words to tag and communicate those concepts to others. In theory we could go all Humpty Dumpty and assign words arbitrarily to the concepts that are useful, without regard to their prior meanings, as long as we can get other people to agree to use each word in the new way -- but that's inefficient, and we lack an Archimedean point to stand on while revising our entire language at once, rather than using one existing word to pull at another, then the revised second word to reshape the first, all from inside.
Because a given socio-environmental context tends to present people with a similar set of problems, most of the time we can rely on a relatively constant, shared set of good-for-all-the-purposes-we're-likely-to-encounter words. The function of the dictionary is to help us out by recording those words and their typical meanings so we can understand people when we don't have time to get clarification from them about a word they used that we don't share. The meanings of words change over time as the problems we need to solve with them change.
But the dictionary just records the solutions that the general public of a given language-user community (or at least the general public as recognized by the dictionary makers, who, as ABW points out, may be unrepresentative) do currently use. If you come up with different concepts to deal with the problems you encounter -- either because you have a creative or insightful new solution, or because you encounter problems different from those of the dictionary-maker's assumed public -- you'll need a different set of word-meanings. If the concept is radically different from the existing ones, you'll probably be better off coining a new word. But if it's not, you may find it best to use an existing word in a different way from the dictionary-writers.
Argumentum ad dictionarium puts this whole process backwards. It takes the dictionary as a fixed, authoritative list of words -- and hence of concepts -- that may permissibly be used. It erases the pragmatic aspect of the dictionary, treating its set of words as being of universal applicability rather than specific to the social world of the dictionary-writers. It then disguises what's really a battle over the recognition of problems and the best concepts to tackle them -- e.g. should we focus on the problem of personal prejudice, or on structural factors that allocate power disproportionately? -- into an argument about definitions, as if an analysis of the word "racism" will tell us how to address race-related problems in our society. (Though people rebutting the argumentum ad dictionarium sometimes fall into this trap too, by misleadingly arguing that the structural perspective is correct because it's the "real definition" of racism, rather than saying that it's the better concept and therefore that's where we should be placing this useful word "racism.")