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A plan is impracticable when you cannot actually do it, or only with huge, almost impossible trouble — because of real things like time, money, tools, or the shape of a place. “Nice idea, but impracticable” means the idea may be clever, even right, and still will not run in the world we have. Compare impractical: an impractical dress is a bad choice — too fancy, too easy to ruin — but you can still wear it. An impracticable plan cannot be put into action. This is a fairly formal adjective, used by teachers, reports, and engineers. Though actually, many people just say not workable or not feasible, and some use impractical for both, which is a bit loose.
不可行的、行不通的（別和「不切實際」搞混）
組成im- (not) + practicable (able to be done).
同家族的字
- practicablethe positive form; im- turns it to “not”
- practicalsame practice family, but practical / impractical is a slightly different pair
- practicethe base; practicable means able to be put into practice
原始詞典資料4 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
impracticableadj/ɪmˈpɹaktɪkəb(ə)l/
- 1
Not practicable; impossible or difficult in practice.
- It has not been used for many years, and although it was impracticable to photograph the engine in the small confines of the shed it was possible to obtain a picture of the plate which it still carries showing the former ownership.
- Prison abolitionists are dismissed as utopians and idealists whose ideas are at best unrealistic and impracticable, and, at worst, mystifying and foolish. This is a measure of how difficult it is to envision a social order that does not rely on the threat of sequestering people in dreadful places designed to separate them from their communities and families.
- 2
impassable (of a passage or road)
- 3
obsoleteunmanageable
- And yet this tough impracticable heart / Is govern'd by a dainty-finger'd girl ; […]
- H. is a person of extraordinary health & vigor, of unerring perception, & equal expression; and yet he is impracticable, and does not flow through his pen or (in any of our legitimate aqueducts) through his tongue.
impracticablenoun/ɪmˈpɹaktɪkəb(ə)l/
- 1
obsoletean unmanageable person
- They were not allowed, of course, to join us in the sitting room, partly that their practice might not be disturbed, but principally, that I was looked upon as an utter impracticable.
- The strict constructionists had dwindled to a few impracticables, headed by John Randolph.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。