Sinew is a waxed thread that divides into 3 or 5 smaller threads. You put the flat thread between your thumbs and pull apart and the sinew will split up. Used for making dreamcatchers, sewing moccasins or any leather item, rattles etc.
The most important evolution in Native American Art & Craft over the past 100 years has been the invention of waxed polyester ’sinew’. Historically sinew which was used for sewing anything and everything, were the tendons off the back leg of a deer or elk. But at some point over the past century someone invented the modern waxed poly sinew which has evolved into a mandatory part of every Native craft box. If you dig deep in any NDN car in the country you will probably find a pair of lost scissors and a roll of sinew.