The word "goodbye" is a contraction of "God be with you"

Say “goodbye” and you are, etymologically, blessing the person you are leaving. The word is a worn-down contraction of “God be with ye” (or “you”), a stock parting blessing in late-medieval English.

The shortening happened in plain sight over the 1500s. The Online Etymology Dictionary traces a form “godbwye” to the 1570s, with the modern shape appearing by the 1590s. Sixteenth-century writing is littered with the intermediate slurrings — God be wy you, God b’uy, God buoye — each a snapshot of a phrase collapsing in real time as people said it fast and often.

The blessing didn’t vanish; it simply contracted until almost no one could see it anymore.

The odd part is the “good-” at the front. The original held “God,” not “good.” The first element was reshaped by analogy with the other farewells crowding the language — “good day,” “good morning,” “good night” — until the divine God quietly became the mundane good. As one account puts it, “‘God’ may also have become ‘good’ because it seemed to go with other expressions like ‘good morning’ and ‘good night.’”

The result is a word most English speakers use a dozen times a day without a flicker of its meaning. A 400-year-old prayer, abbreviated past recognition, survives as the most ordinary thing we say.