Example: The sore on his lip was as plain as the nose on your face. (Source.)
Origins: Shakespeare’s The Two Gentlemen of Verona, Thomas Hardy’s Pair Blue Eyes, 1873. (Origins.)
- as plain as the snout on a sow
- as obvious as the horn on a rhino
- as evident as a turret on a tank
- as patent as whiskers on a chin
- as sure as cute on a kitten
Discussion: Although the idiom is mostly about everyday obviousness, you might also read it to be about plainness and certainty.