A region of Klatch where the people are thought of as being slow-witted, dim and hard of thinking. Just as Americans tell "stupid" jokes about the Poles, the French tell them about the Belgians and the British have a thing about the Irish, no doubt Ur occupies the same position for Klatchians.
Lord Vetinari, when using the Gulli, Gulli and Beti identity, stated that Ur had a university (where else do you think Al learned what a donkey looks like?). However, this may only have been a ruse, when Fred Colon mentioned that the Unseen University had a magic carpet (He called it simply the University).
When Fred Colon was asked where he came from, he began with the time-gaining word "Er...". The Klatchians thought Fred Colon was from Ur, which allayed their suspicions and ruled out his being an Ankhian spy, as no spy could possibly be that stupid.
This would appear to be a reference to the biblical city of Ur, birthplace of Abraham.