Not a free word now; it survives chiefly in 桢干 (a pillar of support, a key figure) and in given names. The right side 贞 is the phonetic, shared with 侦 and 帧.
memory
Picture the upright end-post that holds the boards while a wall is rammed: that load-bearing 木 post is why 桢 came to mean a person everything leans on.
Supplies the sound. 贞 (upright, steadfast) gives the zhēn reading and may add a faint flavour of 'firm and upright', fitting a supporting post; it heads the family with 侦 and 帧.
