mid-14c., rascaile "people of the lowest class, the general mass; rabble or foot-soldiers of an army" (senses now obsolete), also singular, "low, tricky, dishonest person," from Old French rascaille "rabble, mob" (12c., Modern French racaille), as Cotgrave's French-English Dictionary (1611) defines it: "the rascality or base and rascall sort, the scumme, dregs, offals, outcasts, of any company."
This is of uncertain origin, perhaps a diminutive from Old French rascler, from Vulgar Latin *rasicare "to scrape" (see rash (n.)) on the notion of "the scrapings." "[U]sed in objurgation with much latitude, and often, like rogue, with slight meaning" [Century Dictionary]. Used also in Middle English of animals unfit to chase as game on account of some quality, especially a lean deer. Also formerly an adjective.