An exclusively Iberian name recorded from the mid-8th century, and frequently throughout the 9th century, predominantly in northwestern Iberia.
Of unknown origin. Suggestions have included:
a masculine derivation from Greek διδαχή (didakhḗ), perhaps by conflation with διάδοχος (diádokhos);
an extension of Latin Didus, Didius (masculine form of Dido) with the Basque suffix -ko-;
a Celtic *Divakos, from *deiwos "god";
derivation from Celtiberian Titiacus, from a presumed *Tritiakos.
Vernacular forms Diaco, Diago by the 10th century, the form Diego is recorded from the late 11th century.