Etymology.Basque suffix of place. Caro Baroja relates it to the Celtic -acus, -aca, by means of which and a patronymic many place names in Gaul would be formed. He also recalls the names ending in -ca, -aca, such as Lesaca, Mundaca, Gabica, Barandica, Ercoreca, which he considers to be similar to those used for several names of Cantabrian towns in ancient texts: Ottaviolca, Vellica, Camarica or Tamarica.... The latter would come from the gentilic name of the Tamarici. Also Arriaka (Alcarria), so similar to the Basque Arriaga. It appears in Roman times. Aga is general to all the Basque toponymy of all the dialects, but it is absent both in speech and in written literature. It seems to come from -a.ko.a > *aka or from having given the determiner -a to the pluraliser -ak once its function in the toponym had been forgotten: Lizarraga, would, according to this supposition, come from Lizarrak.a. The Vardulians already had Gabalaeca and the Basques had Jakka and Tarraga. In the 10th and 11th centuries, forms such as Amezaba for Amezaga appear. See -IKA, -KA. But this meaning of plurality seems to be contradicted in favour of an indication of place as in Andrearriaga, toponym of Oiartzun (Gipuzkoa), which undoubtedly alludes to the famous stone with an inscription from Roman times or, more clearly, to the famous stone with an inscription from Roman times, as Michelena indicates in the surname Arespacochaga (Bizkaia 1468), from bak(h)oitz, bak(h)otx, with the sense of ‘unique’, ‘solitary’ and as the same author attests, it is very frequent in toponyms and surnames with radicals that are difficult to interpret: Adurriaga, Alzaga, Arteaga, Arteaga, Barinaga, Valenciaga, Troteaga, Nabuchaga, etc. He also mentions cases with personal names such as Catalinaga, Ellacuriaga, Sistiaga and Suinaga. Also with names of saints: Doneperiaga. Don Miliaga, ‘San Millán’ in Berástegui. It is therefore more probable that the suffix -aga denotes place in all or almost all those names where it is present.
Refs: Michelena, L. Basque surnames. S. S. 1955, 36-37 Caro Baroja, J.: Materiales para una historia de lengua vasca en su relación con la latina. Salamanca, 1946.