Conceptual Engineering, the practice of stipulating a change in the meaning of a word in order to improve it in some fashion, for some end, has proved a popular topic among philosophers of language in recent times. Deutsch (Philos Stud 177:3935–3957, 2020) has argued that it has received an undue degree of interest since its implementation falls onto one of the horns of a dilemma: either the change to be effected is in the global semantic meaning of the given word/concept, (...) in which case it is infeasible, or it is merely in the local speaker-meaning, in which case it is largely trivial. I suggest, however, that there is a deeper, more troubling, objection to Conceptual Engineering. My criticism does not strictly take the form of a dilemma, since some cases may fall upon both horns at once and others may be able to squeeze through the gap, but it does involve highlighting two potential weaknesses of the project: one of those, as with Deutsch, is the danger of triviality, although it is not concerned only with the division between semantic and speaker meaning. It seems clear that there are cases where a particular authority or legislature has the capacity to alter a word’s legal meaning; others where certain groups may discuss the best way to use a technical term, each putting forward their preferred suggestions; and individual speakers are at liberty to choose to use a word with a particular intention. None of this is new, nor controversial, and, thus, is largely trivial. In all cases, but particularly where meanings are manipulated without due authority, inexplicitly, or in order to further a particular theoretical conception, however, there is a strong chance that the engineer will be led to commit one of the fallacies of language to which philosophy is particularly vulnerable (Hinton Evaluating the language of argument, Springer, Cham, 2021). These include fallacies of vagueness, equivocation, and persuasive definition. In this paper, I use examples to illustrate the likely fallaciousness of conceptual redefinitions and highlight in particular the ‘Philosophical Fallacy’, outlined by Nelson (A theory of philosophical fallacies.Translated by Leal F, Carus D. Springer, Cham 2016). I conclude that, as a method for philosophers, Conceptual Engineering is, in broad terms, either trivial or fallacious. (shrink)