What is the origin of the word “goodbye”?

Question

I've heard that goodbye comes from God be with you. Is that true? If so how did it become good? Did goodbye always have the same meaning it has now?

Answer 1

The OED also says that this is a contracted form of "God be with you", and explains the God-to-good transition thusly (emphasis mine):

A contraction of the phrase God be with you (or ye); see GOD n. 8. The substitution of good- for God may have been due to association with such formulas of leave-taking as good day, good night, etc.

In a quote from 1659, we still have "God":

But mum for that, his strength will scarce supply His Back to the Balcona, so God b' wy.

And then by 1694, we have transitioned to "good":

He flings up his tail..and so bids us good-b'wy.

Answer 2

The Merriam-webster confirms what you are writing in your question:

Etymology: alteration of God be with you Date: circa 1580

Note (Common Errors in English Usage):

GOOD-BY/GOOD-BYE/GOODBY/GOODBYE

All of these spellings are legitimate; but if you want to go with the most popular one, it’s “goodbye.”
This spelling has the advantage of recalling the word’s origins in phrases like “God be with ye.”

Answer 3

goodbye, good bye, good-by, 1590s, from godbwye (1570s), itself a contraction of God be with ye (late 14c.), influenced by good day, good evening, etc.

Answer 4

Quoting Urban dictionary.com:

kthxbye: shortening of "k thx bye". The K is short for OK, which is short for oll korrect, which is a facetious alteration of All Correct. thx is short for thanx which is a facetious alteration of thanks which is short for thank you. Bye is short for goodbye, which is an alteration of alteration of God be with you. kthxbye is the pinnacle of English's advancement, shortening All correct, Thank you, God be with you. into seven lowercase letters.