It seems that Theneuille (and other French place names as Théneuil, e.g.) comes from the Gallic root
tanno-, "kind of green oak". This root is present in Breton,
tannen, "oak"; in old Irish,
tinne or
teine, "holly tree"; in Cornish,
glastannen, "oak" or "holm oak"
More information, and controversy about some related words, at:
http://monsu.desiderio.free.fr/jardin/chene.html (in French)
Lumia
http://onomastica.mailcatala.com