(as) plain as a pikestaff
US /əz pleɪn əz ə ˈpaɪk.stæf/
UK /əz pleɪn əz ə ˈpaɪk.stæf/

1.
一清二楚, 顯而易見
very obvious and easy to understand
:
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His guilt was as plain as a pikestaff.
他的罪行一清二楚。
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The instructions were as plain as a pikestaff, so I don't know why he made a mistake.
說明一清二楚,所以我不知道他為什麼會犯錯。