good-bye: [16] The good of good-bye was originally God. The expression is a contraction of God be with you, a form of farewell first recorded in the late 16th century. Its gradual reduction can be traced through a series of metamorphoses (Shakespeare, for instance, had God be wy you and God buy’ ye), and it did not reach modern good-bye until the 18th century. The substitution of good for God seems to have been mainly due to the influence of such phrases as good day and good night. => god[good-bye etymology, good-bye origin, 英语词源]