"small saddle horse let out for hire," c. 1300, from place name Hackney (late 12c.), Old English Hacan ieg "Haca's Isle" (or possibly "Hook Island"), the "isle" element here meaning dry land in a marsh. Now well within London, it once was pastoral and horses apparently were kept there. Hence the use for riding horses, with subsequent deterioration of sense (see hack (n.2)). Old French haquenée "ambling nag" is an English loan-word.