一经
yījīng · HSK 7
Once
一 (one) + 经 (pass through) = once it passes through, i.e. once X happens
一经 marks a trigger: once an action takes place, a result follows. 一 carries the once and 经 means to go through or undergo, so literally one pass-through.
一 — single horizontal stroke
一 is one of the simplest pictographs in Chinese, depicting a single horizontal line to represent the number one. It has retained essentially the same form since oracle bone script over 3,000 years ago. By extension it also means 'first,' 'whole,' or 'same.'
经 — silk radical 纟 + 巠 (underground stream)
Originally written 經, combining the silk radical 糸 with 巠, depicting vertical threads on a loom. The core meaning was 'warp threads,' which extended to 'pass through,' 'classic text,' and 'manage.' The phonetic 巠 also suggests the sound.