约定俗成
yuē dìng sú chéng · HSK 7
established by popular usage
🤝 约定 agree-settle + 俗成 custom-becomes = what people settle on turns into the norm
约 pact + 定 fix, then 俗 custom + 成 form: a usage nobody officially decreed, yet everyone settled into, so it hardened into accepted convention. Think of how a word's meaning gets fixed just because people keep using it that way.
约 — silk radical 糹/纟 + ladle 勺
约 combines the silk radical 糹 with 勺 (ladle), which provides the sound. Originally it referred to binding or tying with silk cords, from which came the meanings 'to bind by agreement,' hence 'pact, appointment,' and later 'approximately' (a loose, tied-up estimate).
定 — roof 宀 + 正 (correct/upright, phonetic)
Originally written with 宀 (roof) over 正 (upright/correct), suggesting a household settled and orderly under its roof. Over time 正 simplified to 㐬-like form at the bottom, and the meaning shifted from 'settled home' to the broader senses of 'fix, stable, decide.'
俗 — person radical 亻 + valley 谷
A semantic-phonetic compound: 亻 (person) gives the meaning and 谷 (gǔ/valley) suggests the sound. Originally referred to the customs and habits of common people living in the valleys, later extending to mean 'common,' 'vulgar,' or 'secular' as opposed to refined or religious.
成 — 戊 (halberd/weapon) + 丁 (nail/man)
Originally a pictograph of a halberd or axe-like weapon, suggesting the act of completing a task by force. Over time it shifted from 'to subdue' to the broader meaning 'to accomplish' or 'to become'. The 丁 element inside also hints at the chéng sound.