Picture something sitting on a shelf, finished, before you ever walked into the shop. Someone else did the cooking, the , the building, and you arrive at the last step: pick it up and use it. Two things follow from that picture. You save the work — and you lose the fit, because the thing was made for anyone, not for you.
The dictionary records it as Middle English redye made, that is ready plus made, so the modern picture is already written into the two words.
字典記錄它來自中古英文的 redye made，也就是 ready（準備好了）加 made（做好了）。現在這個「先做好、等你拿」的畫面，其實字面上早就寫著。
Wiktionary entry supplied in packet (etymology_text: From Middle English redye made, equivalent to ready + made), source_ref kaikki:3f6f69a190036ae4:1499757284:7121
Two branches
先做好的東西
這一家就是貨架上那個成品本身，先講形容詞，再講把它當名詞用的情況。
adjective · B1
Already made or prepared before you get it, so you can use it straight away instead of making it yourself. Used about food, clothes, furniture and other goods; ready-made clothes are made in standard sizes rather than made to fit one person.
現成的；已經做好的
先想好的想法
把貨架上的成品換成一個想法，畫面完全不用改：它在你需要之前就完成了，你只是拿來用。
adjective · B2
Already there for you to use, so you do not have to think of it yourself. Used about ideas, excuses, answers, plans and groups of people that a situation simply hands you — and sometimes about opinions people repeat without thinking.
現成的（理由、想法、答案）