Albert Dauzat in, Dictionnaire étymologique des noms de famille st prénoms de France claims that the version, Jaunay, represents the name of a locality possibly in the Département of Vienne. The village of Jaunay-clan is located in this region and thus I assume that Dauzat is referring to this village. French place names ending in ay are often located in the vicinity of water. Jaunay-clan is on the River Clain.
The locality Jaunay has been so named since the life of St Leger [ca616 - 678AD] and without doubt existed in the merovingien era [fall of Roman Empire until 750AD] according to the director of the Vienne archives. He considers the earliest written reference approximating the current form occured in 1162 as Jaunaicum. L Résdet, Topographical dictionary of the Département of Vienne 1881 p 216 indicates several ancient Roman references to the modern town of Jaunay-clan written as Gelnacum, Gallinacum, Jalniacus, Janizas, Jazenas, Galnaica and Galinaco.
Still in the general region we have a river named Jaunay. It is unlikely the town gave the river its name as they a quite separate. The river itself has generated many place names using Jaunay especially bridges and streets.
The River Jaunay has its origin in the region to the south of the town of le Poire-sur-Vie in Vendése, a region of France which widely features the name, Jaunay, both as a family and place name. The valley of the source is called the Val du Jaunay.
Short by river standards, a mere 40 km long, the Jaunay's main claim to fame is a popular man-made lake, Lac du Jaunay, which was created by damming the river just east of the village of la Chaize-Giraud.
The lake is a mecca for camping holidays and several holiday sites can be found on its shores. The largest being Camping Lac du Jaunay which has its own site on the Internet.

Lac du Jaunay

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