俗
ゾク
vulgar, customs, manners, worldliness, mundane things
JLPT1 G7 S9 F1610
As per 谷# (cavity) + 人 person → person who figuratively falls into a hole (compare 禍, 陷, and 凶), becoming accustomed to *worldly*, *vulgar* ways → *customs*; *manners*.
[Full Kanji Etymology List]