"Hair Nude" is photographic pornography
show pubic hair
. The Japanese
law, now widely flouted, proscribes pornographic depictions that show pubic hair -- that's the criterion for unacceptable material. Shaved beaver
s or pre-puberty snatch shot
s were fine. I don't know whether drawings of pubes were strictly illegal, but many of the pornographic comic books popular there would have the naughty bits
drawn as if they'd been blotted out by a censor
I lived in Japan in the 1980s and early 1990s, and IIRC it was about 1988 or 1989 that art photographers started testing that law. Many black and white "artsy" coffee table books featuring one nubile actress or another were printed, much to the delight of many living there.
When I first went to Japan, I saw imported porn mags (Penthouse and Playboy) that had been gone over by a censor: they'd use an abrasive eraser to remove pubic hair from the crotch shots. Porn videos -- and mainstream movies that happened to have frontal nudity -- also featured scrambled or mosaic'd areas obscuring pubic hair.
Oddly enough, it was legal to show tits on TV, and they were common on evening men's variety shows -- 10:00 or 11:00 PM, on standard broadcast channels.