In Mandarin Chinese, 阴 (Pinyin: yīn 🔊) is an adjective meaning “cloudy”, “overcast”, “dark”, “hidden”, “secret”, “insidious”, “negative” etc., or a noun for “the moon”, “shade”, “shadow” or “back”. All these meanings are related to Yin, “a principle in Chinese and related East Asian philosophies associated with dark, cool, female, etc. elements of the natural world”. According to Wikipedia,
Yin is characterized as slow, soft, yielding, diffuse, cold, wet, and passive; and is associated with water, earth, the moon, femininity, and nighttime.阴 is also a word for human “private parts”, that is, anus and (both female and male) genitals — maybe because they are supposed to be hidden?
阴 is a simplified form of the traditional character 陰, which is a combination of radicals 阝 (itself a simplification of 阜, “hill” or “mound”) and 侌 (which has the same meaning as the whole 陰), thus translated as “dark side of a hill”. The 侌 got simplified to one aspect of Yin, 月 “the moon”.