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|Related names||Jacob, Jacobo, Diogo, Tiago, James, Santiago|
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Diego is a Spanish and Italian male name derived from the Hebrew Yaʻăqōbh (Jacob), via the name of Saint James the Greater (Sant Yago), re-analysed as Santiago and San Diego. The assimilation of the final t of Sant into the name is attested in other saints' names, including Spanish and Portuguese Telmo 'Elmo[disambiguation needed]' (< Sant Elmo) and Slovenian Tilen 'Giles' (< Šent Ilen).
The patronymic for Diego is Díaz in Castillian (used for example by Rodrigo Díaz de Vivar, better known as El Cid) and Dias in Portuguese. Like many patronymics, these have become common surnames in Spanish- and Portuguese-speaking regions. The form Diéguez is much less common; Diegues can be found in Portuguese-speaking countries. de Diego and Diego can also be found as surnames.
Medieval usage of different forms
During Medieval times, the names "Sant Yago", "Diago" and "Diego" seem to have coexisted. "Sant Yago" is used, for example, in a letter by James II of Aragon dated 1300: "[...] maestro de la cavalleria de Sant Yago et de la dita orden [...]".
"Diago" is recorded, for example, in "Et fue a casa del Rey. e mostrolo a don diago que era adelantado del Rey" (Fuero de Burgos, ca. 1240)
The name Didacus
Diego was translated into Latin as "Didacus" ("learned person"), which is retrospectively traced to the Greek Διδάχος, Didákhos, "teacher", from διδάσκειν, didáskein, "to teach". Thus, for example, the usual English language name for San Diego de Alcalá (after whom San Diego, California is named) is "Didacus of Alcalá".
This form, and its Spanish equivalent "Didaco", were most likely created in retrospect (that is, to translate Diego into Latin, as opposed to being the source of the name Diego). There are no mentions of Spanish people named Didacus during the Middle Ages. During those times, it was common practice to Latinize existing names, as in Ludovicus for Ludwig (Luis in Spanish).
Even so, some have insisted on deriving Diego from Didacus; nineteenth-century Spanish author Benito Pérez Galdós has a passage in his National Episodes Series 4 (Narváez) that reads:
Su nombre es Didaco o Yago, aunque vulgarmente lo llaman Diego. (His name is Didaco or Yago, although they vulgarly call him Diego.)
In today's Spanish-speaking countries, Diego and Santiago are common as given names; Diego, Santiago and Sandiego are found as surnames. The forms Tiago, Thiago, Diago and Diogo are seen mostly in Portuguese speaking (lusophone) countries.
"Diego tiene chulería y ese punto de alegría", The Ketchup in "Asereje"