blackguard (n.) Look up blackguard at
1530s, "scullion, kitchen knave," of uncertain origin. Perhaps in reference to military units or attendans 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."