勿
モチ, ブツ, ボツ, なか.れ, なし
not, must not, do not, be not
JLPT0 G13 S0 F5000
A depiction of curved pieces of cloth dimly visible in the liquid in which they are being soaked → *negation* (← negativity likened to darkness) → *prohibition*.
[Full Kanji Etymology List]