Japanese for "strange foreigner". The paradoxical part is
that the meaning is the exact opposite of what you'd think!
The Japanese expect foreigners to be weird, so going around
speaking English, shaking hands, not knowing how to use
chopsticks and so on is not considered strange: quite the
contrary, it's only to be expected, and the Japanese are usually
extremely accommodating and more than glad to offer a friendly
lecture on how we Japanese do things.
But the henna gaijin do not fit the mold -- they creep
out the Japanese by acting too Japanese, even though they
blatantly look like foreigners. Henna gaijin can speak
Japanese, use chopsticks, eat natto with relish, know more about
Buddhism and garden landscaping than most Japanese people, and even
use the elevators correctly.
They blur the distinction between gaijin
and nihonjin, raising all sorts of the disquieting
questions about what it really means to be Japanese.
Surely it cannot be merely a question of physical
appearance, now can it?
And the sad part is that someone who looks like a foreigner can
never, ever become more than a henna gaijin...