1520s, "crafty, cunning," as foxes are, from fox (n.) + -y (2). Middle English had foxish in this sense (late 14c.). Of colors, stains, tints, etc. from 18c. Meaning "attractive" (of a woman) is from 1895, American English slang. Related: Foxiness.
The compiler of the "Brut" chronicle, complaining of English fashions in the time of Edward III, notes that þe wemmen ... were so strete cloþed þat þey lete hange fox tailes sawyd beneþe with-inforþ hire cloþis forto hele and heyde hire ars. That is, the women's clothing was so tight/scanty "that they let hang fox tails sewn inside their clothes at the back to ... hide their arses," the which behavior, he writes, perauenture afterward brougte forþe & encausid many mys-happis & mischeuys in þe reaume of Engelond.