Ahhh! I get it! Dyeing the wool before it is spun (i.e., dyed-in-the-wool) is the best way to ensure the color doesn’t fade once the wool is spun into yarn or thread, and then fashioned into a garment worn around town. The literal meaning of the phrase is simply a description of when the dye is added.
The phrase has evolved over time to mean something metaphorical as well. We use this expression “dyed-in-the-wool” to indicate a person who is committed to an ideal so deeply that it has become a permanent part of their being, as if they had been “dyed” that way prior to being “formed.” For example, I would say, “My dear Arabella is a dyed-in-the-wool romantic.”
Here, Arabella is rinsing some Corriedale roving that she dyed while it was “in the wool.”

I have been anxious to get my hands into the dye pot, and finally had opportunity at Arabella’s. She had received a 3-lb bump of lovely Corriedale, and shared a generous portion with me as a birthday present.
A “bump” is simply a bunch of roving, usually 1 to 3 pounds in weight.

Here is what our lovely and generous sheep might have looked like when the wool was still a part of him!

Gorgeous and generous sheep! They have been supplying us with fiber for our clothes, blankets, coverlets, coats and carpets for centuries!