"worn-out horse," late 14c., apparently originally "cart horse," a word of uncertain origin. Barnhart and Century Dictionary suggests a variant of yaid, yald "whore," literally "mare" (c. 1400), from a Scandinavian source akin to Old Norse jalda "mare," and ultimately from Finno-Ugric (compare Mordvin al'd'a "mare"). But OED finds the assumption of a Scandinavian connection "without reason." As a term of abuse for a woman, it dates from 1550s; in early use also of mean or worthless men, and sometimes simply "a young woman."