Some people wear t-shirts that point to their crotch. Some wear ones that point to their breasts. Reimu wears one that points to her armpits, of course.
In Chinese, radical "moon" is used for body parts, and gives the meaning. The phonic part "night", which is a more common character, gives the pronunciation to "armpit", as they sound exactly the same. A good number of compound characters follow this rule, one part for meaning and one part for sound. It's like how many English words have a prefix followed by some common combination of letters.