1530s, "scullion, kitchen knave," of uncertain origin. Perhaps in reference to military units or attendants so called for the color of their dress or their character; more likely originally a mock-military reference to scullions and kitchen-knaves of noble households, of black-liveried personal guards, and of shoeblacks. See black (adj.) + guard (n.). By 1736, sense had emerged of "one of the idle criminal class; man of coarse and offensive manners." Hence the adjectival use (1784), "of low or worthless character."
"dune," Old English sondhyllas (plural); see sand (n.) + hill (n.). For sand-hiller "poor white of Georgia or South Carolina" (by 1848) see cracker (n.2). Earlier it meant "blackguard, user of foul language" (by 1813) in the dialect of Durham and Yorkshire, according to contemporary sources probably from Sandhill in Newcastle.