From Old French Lucie, the vernacular form of Lucia. It is sometimes assumed that Lucy is a pet form of Lucinda, but there is no etymological justification for this assumption. It was in fairly widespread use in the Middle Ages, and increased greatly in popularity in the 18th century and again in the 1990s. In Ireland it serves as an Anglicized form of Irish Luíseach.
Spanish, Portuguese, and Galician: either a habitational name from any of the numerous places in Portugal and Galicia named Redondo, or a nickname for a plump person, also from redondo ‘round’ (Latin rotundus).