So Ur-Lisp would be the old original Lisp that is predecessor to all other Lisps.
Other examples include Urrind (Rind == Bovine) which would be the predecessor of all species of bovine. Urzeit (Zeit == time), which is a generic very ancient time period before all humans (where the Urrind or even the dinosaurs might have lived). Ur-Computer might be one of the first computers, like the original Zuse or ENIAC.
Goethe believed he had seen the Urpflanze, or plant-archetype, while traveling in Italy.
Briefadel -- people whose families were ennobled in modern times.
Edit: "modern times" in this context is "since the 14th century".
https://en.m.wiktionary.org/wiki/ur-
Now, to your original question. No idea which of the early Lisps qualify exactly as such, or until when.
From Middle High German ur-, from Old High German ur-, ir- (“thoroughly”), from Proto-Germanic *uz- (“out”).