"young kangaroo," 1839, sometimes said to be from a native Australian word joè, but more recently often said to be of unknown origin. Perhaps an extended use of Joey, the familiar form of the male proper name Joseph, for which Partridge lists many common or coarse meanings in 20c. Australian slang. Farmer and Henley ("Slang and Its Analogues") quote an 1887 article on "Australian Colloquialisms":
JOEY is a familiar name for anything young or small, and is applied indifferently to a puppy, or a kitten, or a child, while a WOOD-AND-WATER-JOEY is a hanger about hotels and a doer of odd jobs.