Word Origin & History
1501, "old or decayed tree stump," of unknown origin. Meaning extended to "small ox or cow" (1549) and by 1614 to other animals and people. Specific Amer.Eng. sense of "smallest of a litter" (esp. of pigs) is attested from 1841. Some see a connection to M.Du. runt "ox," but OED thinks this unlikely,
and pronounces the word "of obscure origin."