Shirakawa yofune' (Sound asleep) means that someone was fast asleep and totally unaware of what was going on around, which is based on the anecdote-- when a lier told that he had been on a journey to Kyoto, and he was asked about the Shirakawa area, he thought he was asked about the Shira-kawa River and told that he was not aware much of the river because he traveled down the river by boat in darkness of night, there, he was caught in his lie.
京都に旅行へ行ったと嘘をつく人が、この白河（白川）の地について聞かれ川のことだと思い、夜に船で通ったのでよく知らないと答え、嘘がばれたという逸話から、熟睡して前後を知らないことを「白河夜船（白川夜船）」という。