Did you know that...?

I'm going to try and do a weekly fact on Middle- Earth. I'll try to stay within the bounds of the trilogy and The Hobbit but I might throw in some stuff from other material (like the Silmarillion). So here goes:

You probably know the country of Rohan is also called the Riddermark but do you know what the words Ridder-mark actually mean?

I'll give you a week to answer.