late 12c., from 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 hackney "small saddle horse let out for hire" (c.1300), with subsequent deterioration of sense (see hack, n.2). And compare French haquenée "ambling nag," an English loan-word.