good-bye:  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
salutation in parting, also goodbye, good bye, good-by, 1590s, from godbwye (1570s), a contraction of God be with ye (late 14c.), influenced by good day, good evening, etc. As a noun from 1570s. Intermediate forms in 16c. include God be wy you, God b'uy, God buoye, God buy, etc.