The penis is translated as "阴茎" (literally "yin stem"), but it is a male organ. In the traditional concepts of yin and yang, "阴" is generally associated with the female sex.
To support the idea, the term "阳具" (literally "yang tool") is also used to describe the penis (and scrotum).
Why is the penis, a male reproductive organ, associated with the word "阴"?