a tough string or thread usually made from the dried intestines of sheep, horses, etc. and used for surgical sutures, for stringing tennis rackets, etc.
See catgut in American Heritage Dictionary 4
(kătˈgŭtˌ)
noun
A tough thin cord made from the treated and stretched intestines of certain animals, especially sheep, and used for stringing musical instruments and tennis rackets and for surgical ligatures.