Sound editor Walter Murch has stated that R2-D2's name originated from when George Lucas heard him say it in an editing bay while working on American Graffiti.
From an old interview:
One day I was mixing the second dialogue premix for reel two of American Graffiti and voice-slated it "R-2, D-2," and George, who's sitting in front working on the script of "Star Wars", suddenly stood up: "What did you say?" "Ummm, I don't know.. R-2, D-2--is that what you mean?" "R2D2!!....What a great name!" he shouted, and went back to writing his script. The rest is history.
I have read rumors regarding the origin of C-3PO's name. The one I keep coming across is
C3PO was named after a post office which is located at reference C3 on a map of Lucas' hometown.
This isn't terribly far-fetched, but I have not been able to find any actual confirmation of this in any interviews, annotated scripts, or behind-the-scene materials.
What is the real origin of C-3PO's name?