late 14c., "nimble, quick, vigorous" (early 14c. as a surname), a Scottish and northern word, perhaps from Old English -ræsc (as in ligræsc "flash of lightning") or one of its Germanic cognates, from Proto-Germanic *raskuz (source also of Middle Low German rasch, Middle Dutch rasc "quick, swift," German rasch "quick, fast"). Related to Old English horsc "quick-witted." Sense of "reckless, impetuous, heedless of consequences" is attested from c. 1500. Related: Rashly; rashness.