As seen in the anime/manga Brook performs a technique called 45 degrees which seems to be funny somehow. It's this some kind of Japanese joke or pun?
Apparently the move is a reference to Micheal Jackson. His dancing style has, at times, included moves where he angles himself further than most people can using his feet as support.
Alot from the Thiller Bark arc references Micheal Jacksons Thiller music video.
Going beyond 45 degrees makes it difficult, if not impossible, not to fall over which is frequently mentioned.
When he needs to get stronger, he seems to think leaning more will somehow make him more useful.
The leaning against a wall makes the move much less impressive but, hey, that makes it sillier.
I realize this is a rather old post, but I thought I'd provide some further input for anyone else wondering about the same thing. Another possible reason this is found funny is because, when pronounced a certain way, "45" in Japanese can sound like "shigo", or "after death" (死後).
To expand on that, the Japanese word for four (四) can be pronounced as "shi", the same as the word for death (死). Completing the phrase, 5 (五) is pronounced as "go", making it so that 45 can sound like "shigo", which, as I said, means "after death". Seeing as it's coming from a talking skeleton, that's pretty hilarious.
4 can also be pronounced "yon," which is the only way I've heard him pronounce it in the subbed version of the anime. That takes away a bit of the hilarity of it, but I believe in the manga they leave it open to different pronunciations. Either way, he is standing at 45 degrees with his face against the wall, so whether it sounds like "after death" or not I'd say it's pretty amusing.
I actually got the idea for this by looking up tetraphobia, or the fear of the number four, on Wikipedia, but I verified it by looking up the translations and pronunciations of them. No specific references (aside from the not always reliable Wikipedia), but you can easily look up the translations and pronunciations to verify for yourself.