俗

Left person radical — the side-form of 人. The indexing element. Puts 俗 in the human-condition family with 仁 (humaneness), 信 (trust), 仪 (manners), 伦 (relations). Custom, vulgarity, and the secular world are all matters of how ordinary people live — the radical anchors the meaning in everyday human society.

Right 谷 supplies the sound — gǔ drifting to sú through historical g/s shift across the 谷 phonetic series. 谷 itself depicts a valley with water flowing out between two ridges. The picture suits: ordinary folk live down in the valleys — common life flows along the low ground. Same phonetic: 浴 (bathe), 欲 (desire).