Goodbye is what is left of “God be with ye.” The Oxford dictionary’s earliest example of the squeezed-down version comes from a letter Gabriel Harvey wrote in 1573, and the long form and the short one lived side by side until about 1800. The “God” probably turned into “good” because of the company the word kept: good day, good night, good morrow, all of them said at the door. Once people stopped hearing the blessing inside it, the phrase lined up with its neighbours and took their shape. Nobody decided that, and nobody had to. The meaning wore off, and the word stayed in the one place it was always for, the moment someone goes where you can’t follow.