Picture a sheet of paper that was written before anyone arrived. It already says what is to be done, step by step, and everyone who walks in works from it. Prescriptive describes anything with the character of that sheet of paper: it lays down what should happen, instead of watching what does happen and writing that down afterwards.
The word comes from Latin praescriptivus, built on praescribere, which carried the idea of directing something in writing beforehand. That is the same picture the modern uses still work from.
這個字來自拉丁文 praescriptivus，而它建立在 praescribere 上，帶有「事先以書面指示」的意思。今天的各種用法，都還在同一張「事先寫好的紙」上面運作。
Wiktionary entry 'prescriptive' (packet etymology_text: 'From Latin praescriptivus (relating to a legal exception), from praescript- (directed in writing), from the verb praescribere'; source_ref kaikki:3f6f69a190036ae4:463242810:8300)
From the picture
法律上因時間而成立的權利
這一支跟上面那張紙的關係比較遠。它的重點不是「有人事先規定」，而是「時間久到法律認帳」。所以不要硬把它想成前兩個意思的比喻，記住它是法律圈裡的一個固定說法比較實在。
adjective · C2
A prescriptive right is a right that someone has simply because it has gone on for so many years that the law now treats it as real.
因長年使用而取得的權利
adjective · C1
Saying how something should or must be done, or what should be done, instead of leaving people free to decide.
規定得很死的；指定做法的
adjective · C1
Stating how a language should be used, instead of describing how people actually use it.
規範式的（語言）