High-twist cloth is as it sounds. A cloth that has yarns that have been twisted more tightly than normal, and often with more yarns that normal. As standard worsted cloth has 2 yarns, commonly referred to as 2-ply. High-twist cloth normally comes in 2-ply, 4-ply, and even 6-ply.
The 6-ply I found was even accompanied by a picture of some biceps alluding to its inherent strength. Made me smile.