from The American Heritage® Dictionary of the English Language, 4th Edition
- interj. Used to express an acknowledgment of parting.
- n. An acknowledgment at parting, especially by saying "goodbye.”
- n. An act of parting or leave-taking: many sad goodbyes.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- interj. farewell; a formula used to another person or persons when the speaker or writer or person addressed is departing.
- n. An utterance of goodbye, the wishing of farewell to someone.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. a farewell remark
Alteration (influenced by good day) of God be with you.(American Heritage® Dictionary of the English Language, Fourth Edition)
From earlier 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, with change of God to good by confusion with good morning, good day, etc. (Wiktionary)