Picture something whose edge you cannot find. The thing is there — a path, a patch of fog, a job, a rule — but when you look for the line that says here it stops, there is no line, only a fade. Ill-defined can be thought of as that missing edge. Sometimes the edge is a real one you would see with your eyes, and sometimes it is made of words.
From the word’s history
Wiktionary records the word as ill + defined. Here ill is the older adverb meaning badly, the same ill as in ill-prepared and ill-timed, and define carries the idea of setting a boundary — so the compound says the boundary was set badly.
來源資料記錄這個字是 ill 加上 defined。這裡的 ill 是舊的副詞用法「不好地」，跟 ill-prepared、ill-timed 是同一個 ill；define 本身帶有「畫出界線」的意思。所以整個字字面上就是「界線畫得不好」，跟上面那個「找不到邊」的畫面剛好對得上。
Evidence packet, Wiktionary entry for ill-defined (source_ref kaikki:3f6f69a190036ae4:1671735184:2507), etymology_text: 'From ill + defined.'
登入就可以看每一支怎麼連回核心畫面。