The only free and comprehensive online etymological dictionary of the Spanish language
izquierda (Adjective) "left"
12th cent. Old Spanish exquerdo. From Basque ezkerdo "left-handed" and thus from ezker(r) "left" but this etymology is only superficially convenient as a suffix -do is of unclear origin. It may not come from ezker(r) at all but esku "half" or Celtic *kerros "left" (though the latter has fell out of scholastic favor in recent years). The details and arguments exceed the scope of this dictionary. For a fuller discussion, see Zytsar (2000).