(as) plain as a pikestaff
US /əz pleɪn əz ə ˈpaɪk.stæf/
UK /əz pleɪn əz ə ˈpaɪk.stæf/
1.
very obvious and easy to understand
:
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His guilt was as plain as a pikestaff.
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The instructions were as plain as a pikestaff, so I don't know why he made a mistake.