egoismo

The half you can cut, and the half you cannot. The -ism- on the end is a real Esperanto suffix and it is doing its ordinary job: egoisto is the person, egoisma the adjective, egoismo the doctrine or the habit. That seam is genuine. The piece in front of it is not Esperanto at all. Ego- is Latin for I, borrowed whole, and it is the only place in the language where that Latin pronoun turns up.

So the word a learner expects does not exist. Esperanto already has three perfectly good ways of saying self - mi for I, mem for the emphatic -self, and si for the reflexive - and it built nothing from any of them here. There is no *miismo, no *memismo, no *siismo. The international word arrived under Rule 15 with the Latin already inside it, and Esperanto took it as it stood, because egoismo is recognisable to a reader of any European language and a home-built alternative would have been recognisable to nobody. Where the language does build on its own roots is one step to the side: memamo is self-love, sinforgeso is self-forgetting, and both are transparent in a way egoismo never will be.

Against memfido, which is the useful contrast.Memfido is self-confidence, and it is admirable. Egoismo is selfishness, and it is not. Both are about the self, both turn up in the same kind of sentence about character, and English keeps them well apart while a learner reaching for either one may grab the wrong half. The test is what the self is doing: memfido is trusting yourself, egoismo is serving yourself at somebody else's cost. Avara, miserly, is the money-shaped neighbour - egoismo is wider, and covers the person who takes the last seat as readily as the one who will not lend a coin.

The philosophical sense survives too. As in English, egoismo names a position in ethics - the claim that self-interest is the proper ground of action - alongside the everyday charge of being selfish. Context carries the weight, and egoisma is nearly always the everyday one. Altruismo is the opposite number, borrowed the same way and with the same fossilised Latin inside it.