What does Santiago mean?
A contraction of 'Sant Yago' — Saint James. Yago (also Iago) is the old Spanish form of Jacobo/James, from the Hebrew Ya'aqov (Jacob). The name honors Santiago the Apostle, patron saint of Spain, whose shrine is at Santiago de Compostela.
What does Diego mean?
A Spanish name of genuinely disputed origin. Traditionally explained as a short form of Santiago (Sant-Yago, 'Saint James', from Hebrew Jacob), but many scholars now derive it instead from the Latin Didacus — itself possibly from the Greek didache 'teaching'. The two theories remain unresolved, so any single 'meaning' should be treated with caution.