Inversion of earlier still as a stock, from stock ("tree-trunk").

stock-still (not comparable)

1. Very still; motionless.

   The team was stock-still in front of the stable.

stock-still (not comparable)

1. (idiomatic) Without motion.

   He lay stock-still among the bodies and blasted equipment.

   • 1897, Sam T. Clover, Paul Travers' Adventures:

     Paul stood stock still; then came a strong impulse to turn and run back.