How do we know it's not more like trying to extrapolate the eternally true meaning of the word "triangle"?

Even without going into the complexities of human minds: do you mean triangle in formal Euclidean geometry, or triangle in the actual spacetime we're living in? The latter concept can become arbitrarily complex as we discover new physics, and the former one is an approximation that's simple because it was selected for simplicity (being easy to use in measuring plots of land and such). Why you expect the situation to be different for "morality"?