While Jeff draws a lot of his yojis from conversation and snatches the rest from the ether (a trick I keep meaning to have him teach me), up until now I've just had to string together random kanji and hope they were a yoji. In fact half of my posts so far haven't even made sense to Japanese people until I popularized them on my hit late-night TV show, "Japanese: You're Doing it Wrong!" This just goes to show that even if you can do a ton of other stuff right (like figure skate, or wear pink gloves), it's all for naught if there's something amiss about that bandana...
Anyway, this yoji is special in that I discovered it while reading Slam Dunk, a manga that a few Japanese acquiantances at an enkai told me is a MUST READ. I've never been a huge fan of basketball and thought it would therefore be a bit boring - and thus perhaps induce today's yoji - but instead have found it both educational and wildly entertaining. This phrase could really be applied to either the protagonist or his rival/teammate, though for different reasons. And while the origin might have something to do with taking a night boat down the ol` white river, the actual meaning deviates substantially from the literal.
1. Sound/fast/dead asleep
2. In a know-it-all manner
If you're just given the barebones definition on this one, it's a little disappointing. After the all, the answer seems to have NOTHING to do with the kanji... until you're treated to this explanatory story.
My take on the story: a man takes a trip to the capital to go sight-seeing, and on returning can't stop telling stories about it. Somebody asks him how Shirakawa (the name of a place) was, and the man - mistaking the name for that of a river since it ends with "kawa" - says "oh - well I crossed that river by boat at night, so I didn't really get to see it..."
Like all jokes, it's a lot better when it doesn't require an explanation. The localized (to America) version might read something like "So, you went to New York City last week! How was the big apple?" "oh - unfortunately it wasn't on display, but they had a lot of other big fruits there..."
I think I might be going for a record for most posts with the "bad jokes" tag. ANYWAY, the "dead asleep" usage comes from the idea that you were so soundly asleep your entire trip, you don't know anything that happened around you, and thus thought that perhaps shirakawa was a river, not a place. Usage on this one is pretty simple. If you missed something due to being dead asleep, just roll out with 白河夜船で、～。 Adding "です" and most other simple constructions will work out just fine. And though I can't be sure, it seems the know-it-all definition works primarily on those who don't actually know it all, so use with care.
When I was a kid, my parents always made me go to church. That place was so boring that I fell into a deep sleep all the time. I was really able to kill time that way.