हिंस्

हिसिँ = ह इ स इ चंद्र बिंदी
हिषिं = ह इ ष इ चंद्र बिंदी
object(hard) object(pointed) inside/surface

रिष्वो हिंस्रः
रिष् = हिंस्रः = ह (exists) रिष्
exists a sharp object pointed inside
hard (ह) sharp (र) object (इ) pointed (ष) inside (म)

could also have been
हिंष + ऋ = hard inward inside pointed superior

commentary
हिसिँ is originally
हिषिं = ह इ ष इ चंद्र बिंदी
hard object pointed object inside
This refers to a pointed stone tool being used on something below.