1590s, "offspring of a European and a black African," from Spanish or Portuguese mulato "of mixed breed," literally "young mule," from mulo "mule," from Latin mulus (fem. mula) "mule" (see mule (n.1)); possibly in reference to hybrid origin of mules (compare Greek hēmi-onos "a mule," literally "a half-ass;" as an adjective, "one of mixed race"). As an adjective from 1670s. Fem. mulatta is attested from 1620s; mulattress from 1805.
American culture, even in its most rigidly segregated precincts, is patently and irrevocably composite. It is, regardless of all the hysterical protestations of those who would have it otherwise, incontestibly mulatto. Indeed, for all their traditional antagonisms and obvious differences, the so-called black and so-called white people of the United States resemble nobody else in the world so much as they resemble each other. [Albert Murray, "The Omni-Americans: Black Experience & American Culture," 1970]
Old English had sunderboren "born of disparate parents."