Laggia is a rare surname, scarcely documented in the classic Italian onomastic repertories, so that the etymology proposed here should be regarded as indicative only. The most likely hypothesis links it to the toponymic group of northern Italy, or to dialect derivations related to the Latin lacus ("lake"). There is also the surname Laggio, attested very sporadically in Friuli (the Carnia area) and in the Cadore, linked to the place name Laggio di Cadore, in the province of Belluno (Veneto).

The variant Laggiard, found in Piedmont, is of particular interest: according to J. Tosti, it is a surname of Occitan origin, characteristic of southern France —especially of Provence, with a concentration in the Bouches-du-Rhône department, in the Aix-en-Provence and Marseille area. It would derive from the Occitan term lag or laï ("lake", "pond") plus the suffix -iard, with the sense of "inhabitant of, or owner of, land near a lake or marshy area".

The presence of Laggiard in Piedmont is explained by the geographical proximity and the historical and linguistic ties between that region and southern France, an area of Occitan-Provençal tradition with intense migratory movement across both slopes of the Alps.

This surname does not appear as an autonomous entry in the classic Italian onomastic repertories consulted, so the etymology proposed should be regarded as indicative only.