expression
fast asleep and unaware of surroundings
Yojijukugo describing someone who is sound asleep and oblivious to what is happening around them. Often used in the phrase 白河夜船を決め込む (to pretend to be fast asleep).
彼は会議中、白河夜船で何も聞いていなかった。
He was fast asleep during the meeting and didn't hear anything.
白河夜船を決め込んで、知らん顔をした。
He pretended to be fast asleep and acted like he didn't know.
expression
know-it-all attitude; pretending to know everything
Rare, possibly archaic sense. Refers to a person who acts as if they know everything, often while actually being ignorant. The connection to the literal meaning is unclear; may derive from a story where someone pretended to have traveled to Kyoto by boat on the Shirakawa River while actually asleep.
彼の白河夜船な態度には腹が立つ。
His know-it-all attitude is annoying.