good-byeyoudaoicibaDictYouDict[good-bye 词源字典]
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.[good-bye etymology, good-bye origin, 英语词源]