I’ve long been suspicious of speeches that revolve around idiosyncratic definitions. I was pleased to find this evening that C. S. Lewis shared this suspicion.
But when we leave the dictionaries we must view all definitions with grave distrust. … The fact that they define it at all is itself a ground for skepticism. Unless we are writing a dictionary, or a text-book of some technical subject, we define our words only because we are in some measure departing from their real current sense. Otherwise there would be no purpose in doing so. This is especially true of negative definitions. Statements that honour, or freedom, or humour, or wealth, ‘does not mean’ this or that are proof that it was beginning to mean, or even had long meant, precisely this or that. … We do not warn pupils that coalbox does not mean a hippopotamus.
… A certain type of writer begins `The essence of poetry is’ or `All vulgarity may be defined as’, and then produces a definition which no one ever though of since the world began, which conforms to no one’s actual usage, and which he himself will probably have forgotten by the end of the month.
From Studies in Words.