My dad is someone who refuses to use the word "goodbye" and would prefer either "good night" or "see you again."
This was brought up during our bible study today and we begin to then ask what is the meaning of the word anyway.
It really surprised us to find out that goodbye is a contraction of God be with you.
From the Wiktionary's section on its etymology, the word came from an 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. Note the change of God to good by confusion with good morning, good day, etc.
In that case, it's actually good to say goodbye, isn't it? I'd be wishing that God will be with you.