Like the title says, it's a Leather Water Sac that can be made from TFC items.
It has 12 uses (which is about 2 thirst bars worth) before you have to fill it up again.   To clarify my view on modpacks, if you are using TFC, you are more then welcome to use the Leather Water Sac. Have fun   A few pics:

  To make one, you need
- 2 x TFC Leathers that need to be cut correctly
- 2 x Wool Yarns (String can not be used anymore)
- 1 x Bladder that you get by killing a sheep, cow or deer
- and any kind of TFC knife (The knife gets the same damage as when you use it with crops)   Recipes First you have to cut the leather Then kill a sheep, cow or deer to get the bladder. Then you can craft it in a workbench like this     Version 3.8.B79.23+   Old version Version 3.6.B79.20+ Version 3.5.B79.16+ Version 3.4.B79.13+ Version 3.3.B79+ Version 3.2.B78_HF7+ Version 3.1.B78 Version 3.0.B78 Version 2.1.b77 Version 2.0.b77     Old

Version 1.5.b77

Version 1.4.b76
Version 1.3.b76
    Recipe Version 1.2.b75

Version 1.1.b75