دارم یک کتابِ pragmatics میخونم (یه مبحثی توی زبان‌شناسیه) توش نوشته:

 Goodbye is a contraction of God be with you and in the Middle Ages was widely used among Christains in a variety of ways and conveying a variety of implictures, including being used as a salutation on a meeting or valedication on parting.