An Asian woman's vagina. Not specific to only Japanese women (only because most people can't tell the difference). This term was started by me and is used in a close circle of Milwaukee friends. Surely someday it will be more widespread. Moreover, the reason for this slang is that usually Asian-American women, especially Asian born women are very sexually uptight (but not pertaining to being a virgin) and never let men get any of that tight poontang. If you think about it, most Japanese Puzzle Boxes usually take at least 10 steps to open it. Some puzzle boxes take up to 72 steps just to get it open and you need instructions. Hence, it is very difficult.
Man this sucks, I have been out on 54 dates with my girlfriend and she still won't let me get in that Japanese Puzzle Box.