Allen,
Thanks. I did some more digging and found what I think is the source for references in modern books: Peter H. Wood, BLACK MAJORITY: NEGROES IN COLONIAL SOUTH CAROLINA FROM 1760 THROUGH THE STONO REBELLION (NOrton, 1976), p. 31, footnote 58. Following a discussion of cow-tending activities by black slaves in colonial South Carolina, he writes, "It is even possible that the word 'cow boy' originated through this set of circumstances." Obviously speculative, but who knows what might pop up in a reading of colonial-era documents in South Carolina.

Woods comment made its way into two more recent history books, as well as the National Park Service website, "Africans in the Low Country."

The word "cow boy" (two words, actually) was per the OED in print in Britain by 1725, so it would not be surprising that it sailed the Atlantic and took root in various regions. At the moment, however, the expression's use in New York to describe Tory rustlers during the Revolution is the eariest published citation. More digging may result in earlier citations.

Dan