also grape-shot, 1747, from grape + shot (n.). So called for its appearance. Originally simply grape (1680s), a collective singular. The whiff of grapeshot was popularized in English from 1837, from Carlyle's history of the French Revolution (in which book it was a chapter title). It seems to be his.
The Nobles of France, valorous, chivalrous as of old, will rally round us with one heart;—and as for this which you call Third Estate, and which we call canaille of unwashed Sansculottes, of Patelins, Scribblers, factious Spouters,—brave Broglie, "with a whiff of grapeshot (salve de canons)," if need be, will give quick account of it. [Carlyle, "French Revolution"]