Out of universe, what is Wookiee fur made of?
This fearsome-looking bunch are all actors in costume, presumably. So what material was used to make those costumes?
According to the traveling Smithsonian Exhibit, Chewbacca's costume was made of yak hair and mohair. Presumably, the other wookiee suits were as well. The exhibit features screen-used costumes and props from all six movies, including one of the original Chewbacca costumes.
This article about the exhibit mentions the costume's materials specifically.
Among the other display descriptions: Amidala's wedding dress was made partly out of an Italian lace bedspread; and Chewbacca's fur is a combination of Yak hair and mohair.
The Smithsonian's page for the exhibition also mentions it:
Yak hair and mohair costume of the towering Wookiee Chewbacca
The Smithsonian's page for the costume itself also mentions it:
Designed by concept artist Ralph McQuarrie and produced by Stuart Freeborn, the expressive costume is made from yak hair and mohair.
National Fiber Technology, the company who supplied the hair/fabric for the wookiee costumes in the prequels stated that 70-80 square feet of hair was used to make each costume. (note and forgive the horrible misspelling of the word "wookiee" throughout the linked article).