(Not me! I was asleep!)
Literally: white – river – night – boat
Alternately: Being sound asleep. Being completely unaware of one’s surroundings. Alternately, affecting a know-it-all attitude despite a lack of actual expertise or experience.
Notes: 河 (kawa) can also be written with the more common character 川. 船 can also be written as 舟, and pronounced either bune or fune (both are pronounced with two syllables, generally rhyming with “moon bay”).
Supposedly this phrase originates with someone who boasted (falsely) about having traveled to the capital in Kyoto, a journey that would have taken them through an area called 白河 (Shirakawa). When asked about Shirakawa, though, they thought it was the name of a river, and claimed to have traveled this river on a boat at night, and as a result they didn’t have much to report. The lie was discovered because of this mistake, and so we have the double meaning of today’s yojijukugo – both the deep sleep that the storyteller claimed, and the pretended knowledge.
(Google Maps locates the 白河 area here in present-day Kyoto.)