late 15c., "mixed-breed dog," from obsolete mong "mixture," from Old English gemong "mingling" (base of among), from Proto-Germanic *mangjan "to knead together" (see mingle). With pejorative suffix -rel. Meaning "person not of pure race" is from 1540s. As an adjective from 1570s.[mongrel etymology, mongrel origin, 英语词源]