白河夜船

Etymology 1
From a folk story in which someone who pretended to have visited Kyōto was asked about Shirakawa (/), misinterpreted it as the name of a river, and answered that he or she didn't know because he or she crossed it on a night boat.

Idiom

 * 1) being fast asleep, dead to the world when sleeping
 * 2) pretending to know everything, a know-it-all manner

Etymology 2
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Shift in reading for the portion.