Oh, and I incorrectly attributed Alan Smithee’s correction of Reality Chuck’s over-narrow definition of “folk etymology” to Reality Chuck himself. Sorry about that.
I believe I have also seen “pp” used when someone is signing on behalf of another person.
e.g. you get a letter by A. N. Executive (or a form letter purporting to be from them) with the signature of A. Peon and “pp A. Executive” below the signature.
No idea where this originates from or if it is common practice.
Given sinjin’s cite from the OED that the first recorded use of “CC” was in 1936, that is, long after the invention of carbon paper, and the evident lack of evidence for the plural explanation, I think we can conclude that “carbon copy” is in fact the origin.
According to Merriam Webster, this is an abbreviation for Latin per procurationem, “by proxy.”
It is also an abbreviation for the musical instruction pianissimo, “'very softly.” Here the doubling indicates an intensification of piano, “softly,” rather than a plural.
After receiving my comeuppance on not only “what I thought I knew” about CC but the terminology I used (though I am glad to be in Ciardi’s company on it), I think any further contributions I make to this thread will be ppp. 