Where it comes from
Sinew is a now-and-then drop from the large grazing animals of the surface, the Reedhorn, the Brackenback and the Morrow, and from the cave hunters Riftstalkers and Earwights deep below. It does not drop every time, so expect to take down a few. See Wildlife.
Fire ruins it. An animal that burns to death leaves no sinew, so kill them with arrows, blades or a blast and not with flames.
What it is for
Sinew goes into the reinforced Bow and arrows, made on a Bow table together with bone and glowseeds.