Tail fat is the fat of some breeds of sheep, especially of fat-tailed sheep. It is fat accumulated in baggy deposits in the hind parts of a sheep on both sides of its tail and on the first 3-5 vertebrae of the tail. The weight of this part of a sheep's anatomy may be up to 30 kg. It is used to accumulate fat for subsequent use during dry seasons, similar to a camel's humps.
It is known under the name kurdyuk in Russian and in Central Asian languages,  derived from the proto-Turkic *kudruk meaning "tail" (cf. kuyruk, "tail", in Turkish, etc.). Tail fat is known in Arabic as لية, (leeyeh, leyyah, or layeh), zaaka in Algeria, kyuruk yag (literally "tail fat") in Turkish, and in Persian as donbeh or dombeh, words which may be found in local food culture and in sheep breeds' names.
The rendered tail fat does not solidify at room temperature and is used in cuisine. The cracklings left after the rendering or frying of kurdyuk may be used as an appetizer.  When being rendered, kurdyuk emits a strong odor, described as "acidy-poisonous". However, it has a rich flavor when ready to eat. In particular it is used to cook kofta, pilav, and other traditional dishes. 
- Jill Tilsley-Benham, ""Sheep with Two Tails: Sheep's Tail-Fat as Cooking Medium in the Middle East", In: Oxford Symposium on Food & Cookery, 1986: The Cooking Medium, p. 48
- Brockhaus and Efron Encyclopedic Dictionary, article "Курдюк"
- Food Culture in Russia and Central Asia By Glenn Randall Mack, Asele Surina ,p. 111
- S. A. Starostin, A. V. Dybo, O. A. Mudrak, "An Etymological Dictionary of Altaic Languages", 2003, Bril