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This question already has an answer here:

I've seen when the word transitioned from Godbwye to goodbwye, but when did it become goodbye?

marked as duplicate by Mitch, MetaEd, mplungjan, Matt E. Эллен, TrevorD Sep 3 '13 at 23:39

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According to etymonline (cited by Jim above), it is originally derived from God be with ye in the late 14th century.

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