TSUMUGU · B1 · 字
俗
俗 ＝ 亻 ＋ 谷
ㄙㄨˊ · sú
noun/adj — custom, the common way; common, popular; vulgar
字源 FORM — what the parts do
亻義people — the crowd of them
IFTgǔ → sú — the g wears to s, rime and tone shift with it
A crowd of people (亻) and 谷 gǔ for the sound, worn to sú. 說文 reads it 習也 — what people grow used to: the customs they keep (風俗), the common and popular (通俗), and from there the merely vulgar (俗氣). It rides the 谷 series: 浴、裕、欲.
故事 STORY — a scene to remember it by
一谷（谷）人家，世世代代照著同一套做——成了風俗。
A valley (谷) full of households doing the same things, age after age — that becomes custom.
字源記憶法
意 · Meanings
(本義)風俗、習俗— custom, the common practice常用
→通俗、世俗— common, popular; worldly常用
⇒俗氣、俗語— vulgar; a colloquial saying
(本義) original meaning · → ⇒ each arrow = one more step of extension