"person or thing regarded with especial liking," 1580s, from French favorit, perhaps via Italian favorito, noun use of past participle of favorire, from favore, from Latin favorem "inclination, partiality, support" (see favor (n.)).
Especially, "a person who gains dominant influence over a superior" (1590s). In racing, "one considered most likely to win," attested from 1813. In 17c.-18c. also "small curl hanging loose upon the temple," a frequent feature of a woman's head dress.
As an adjective, "regarded with particular liking, esteem, or preference," by 1711. Favorite son in the figurative sense "noted man who is particularly popular and boasted of in his native area" is by 1788.
As a corresponding noun in the sense of "person who promotes the interests of another," Latin had fautor, hence Old French fauteur, Middle English fautour "an adherent, supporter, follower" (mid-14c.), but it has perished along with its fem. form, fautress.