Know how to use "fewer" and "less"? Find out.
c.1500, "old or decayed tree stump," of unknown origin. Meaning extended to "small ox or cow" (1540s) and by 1610s generally to undersized animals and people. Specific American English sense of "smallest of a litter" (especially of pigs) is attested from 1841. Some see a connection to Middle Dutch runt "ox," but OED thinks this unlikely, and pronounces the word "of obscure origin." Related: Runty (1807).