Terdec is not a place name but probably short for Latin terdeciens ‘thirteen times’.  It occurs in the Cosmography's phrase et Sodisinā Cled Terdec, which is quite close to Latin et sodes insulae Cled terdeciens ‘and if you please islands Clyde thirteen times’. Islands in the Clyde estuary do, as it happens, add up to thirteen if you count just those with signs of human habitation and exclude rocky islets.