Sorry, Mac

Sorry, Mac.

What is the origin of the phrase?

The plain English meaning is, "I'm sorry, fella." The words Mac, fella, pal, chum, are ways of saying, either in a friendly manner or not, "You, Whatever-your-name-is." The Oxford English Dictionary says "[Mac:]Used as a familiar form of address to a (male) stranger", and other dictionaries say the same thing in different words.
My guess (an easy one) is that it started someplace where men of Irish or Scottish extraction were commonly encountered.
SS

That's a good theory.

17C+ Celtic Irishman or Scottish man. 1910s+ (U.S.) general term of greeting with no specific ref. to Scottish men implied. Irish/Gaelic mac:son. Cassell's Dictionary of Slang by Jonathon Green (Wellington House, London, 1998). Page 757.