"Ur" is a German prefix meaning "old", "original", "predecessor".
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.
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.