This means to think that the oldest meaning of a word is the true meaning. For example, originally, the word incel simply meant what is says, involuntary celibate. It applied to both genders and simply meant someone looking for a sexual relationship (in whatever form) and not being able to find one. But nowadays, it’s come to mean someone following a hateful philosophy and is largely negative. So if a person was technically involuntary celibate, but not hateful, then it would be a fallacy for them to refer to themselves in that way, and it’s also a fallacy to think about someone that way (I’m talking about the morphed word ‘incel’, it’s still kind of OK to use the full form, but it might need a bit of explanation if you’re male).