Goodbye
来自Big Physics
late 16th century: contraction of God be with you!, with good substituted on the pattern of phrases such as good morning .
wiktionary
From earlier goodby, Godby, Godby'e, Godbwye, God b'w'y, God bwy yee, God buy you, God be wi' you, each a progressively shorter contraction of God be with you. Note the change of God to good by confusion with good morning, good day, etc.