歯
シ, よわい, は, よわ.い, よわい.する
tooth, cog
JLPT3 G3 S12 F1106
A depiction of front teeth. 齒 adds 止# (stand straight) → straight line of *teeth* → *line up* → *age* (← aging ← disappearance of teeth). For the latter meaning, compare 齡.
[Full Kanji Etymology List]