Using 'selfish' to mean maximising your own utility function also bleeds meaning out of things: by what possible definition would someone not be selfish here?

Someone can fail to maximize their own utility function due to akrasia, irrationality, or incorrect information. (But I agree that “selfish” is an extremely poor choice for a word for that. See e.g. this about “sacrificing one's own happiness for the sake of others” vs “gaining one's happiness through the happiness of others”.)