ninth of the ten Heavenly Stems 十天干[shi2 tian1 gan1]; ninth in order;
Pictograph of a warp beam used for holding silk in, also known as a spool. The modern meaning "9th heavenly stem" is a phonetic loan.
《說文》：“壬，位北方也。陰極陽生，故《易》曰：‘龍戰于野。’戰者，接也。象人褢妊之形。承亥壬以子，生之敘也。與巫同意。壬承辛，象人脛。脛，任體也。”
thread spool