It’s called Stator Lacing and Tie Cord. In another life, we used it to bind rewound coils of wire for motor stators and armatures. It is a braided flat cord (cotton?) that was coated with wax, or something resembling wax. That was 45 years ago and I still have a partial spool of it. You could also wax your own thread with beeswax and get similar results.
