This article's factual accuracy is disputed. (May 2021)
|Related names||Diogo, Tiago|
The name has long been interpreted as reanalysis of Santiago, from older Sant Yago "Saint Jacob," in English known as Saint James, as San-Tiago. This has been the standard interpretation of the name since at least the 19th century, it is so reported by Robert Southey (1808) and by Apolinar Rato y Hevia (1891). The suggestion that this identification may be folk etymological, i.e. a name Didacus, Diego of unknown origin would at a later time have been identified with Jacobo, is made by Buchholtz (1894), even though this possibility is judged as improbable by the author himself.
In the later 20th century, the traditional identification of Diego=Jacobo has come to be seen as untenable. Malkiel (1975) calls the equation an "odd couple" (extraña pareja). The name Didacus, while unattested in anqituity, predates the earliest record of the form Diego. The oldest record for Didacus according to Floriano (1949) dates to 747, with numerous further records during the 9th century. Becker (2009) argues against possible derivation from the Greek name Diadochus, but also against suggestions of Basque and Celtic derivations. 
Didacus is recorded in the forms Diaco, Diago in the 10th century. The form Diego is first recorded in the late 11th century. Its original derivation from Didacus is uncertain, among other things because the shift from -ía- to -ié- is unexplained (Becker 2009:386). The name Diego Gonzalez is given to a character in the Cantar de mio Cid, a poem of the 12th century. It has been argued on metrical grounds that the name Diego in the Cantar represents an original Díago.
Medieval bearers of the name, such as Diego de Acebo (d. 1207), are recorded as Didacus in contemporary sources. Diego becomes the standard form of the name in the 14th century, and it is frequently given in the 16th century, e.g. Diego Laynez, 1512–1565. The city of San Diego was named for the flagship of Sebastián Vizcaíno (1602), which was itself named for Didacus of Alcalá (d. 1463).
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.
"Diego" as a generic name or term for a Spaniard is documented from around 1615. The term "Dago" as a generic name for Spaniards is recorded in the 19th century and may possibly be a derivation from Diego. By the early 20th century, the term dago was extended as an ethnic slur applied chiefly to Italian Americans, besides also for anyone of Spanish or Portuguese descent.
- Lidia Becker, Hispano-romanisches Namenbuch: Untersuchung der Personennamen vorrömischer, griechischer und lateinisch-romanischer Etymologie auf der Iberischen Halbinsel im Mittelalter (6.–12. Jahrhundert) (De Gruyter, 2009), pp. 385–392.
- Robert Southey, Chronicle of the Cid (1808), footnote p. 379.
- Apolinar Rato y Hevia [1830-1894], Vocabulario de las palabras y frases bables, Madrid (1891): "Yago, m. n. de v. Tiago, Jacome, Jacobo, Diego. De todos estos modos se decia Santiago."
- H. Buchholtz, "Der Name Diego" in Archiv für das Studium der neueren Sprachen und Literaturen 93/94 (1894), 274–278.
- Yakov Malkiel, "Espanol y portugues antiguos Diago, Diego y Diogo Entornoala hipercaracterizacion interna y externa" in Medioevo Romanzo 2 (1975), 177-192, cited after Becker (2009), 386, fn. 278.
- Antonio Cristino Floriano Diplomática española del período Astur: estudio de las fuentes documentales del Reino de Asturias (718-910) (1949), cited after Becker (2009), p. 387.
- v. 3646 Martín Antolínez e Diego Gonçález firiéronse de las lanças
- Becker (2009:386).
- González, Félix Rodríguez (1996). Spanish Loanwords in the English Language: A Tendency Towards Hegemony Reversal. Walter de Gruyter. p. 115. ISBN 9783110148459. Retrieved 15 February 2013.