sharp, sharp-pointed, sharpen
剡 centers on making something sharp or pointed, whether describing a blade's edge or the act of sharpening it.
剡 combines 炎 (flame) with 刂 (knife), likely suggesting a blade sharpened by fire or a sharp, pointed flame. The exact historical development is uncertain.
Flames 炎 beside a knife 刂: imagine heating a blade in fire until it becomes sharp and pointed.
For エン, picture the sharp end of a flame: end -> エン, and the flame sharpens the knife.