From Yin &Yang and the I-Ching written by Kelly L. Ross, Ph.D.:
Yin originally meant “shady, secret, dark, mysterious, cold.” It thus could mean the shaded, north side of a mountain or the shaded, south bank of a river.
Yang in turn meant “clear, bright, the sun, heat,” the opposite of yin and so the lit, south side of a mountain or the lit, north bank of a river.
From these basic opposites, a complete system of opposites was elaborated.
Yin represents everything about the world that is dark, hidden, passive, receptive, yielding, cool, soft, and feminine.
Yang represents everything about the world that is illuminated, evident, active, aggressive, controlling, hot, hard, and masculine.
Everything in the world can be identified with either yin or yang. Earth is the ultimate yin object. Heaven is the ultimate yang object. Of the two basic Chinese “Ways,” Confucianism is identified with the yang aspect, Taoism with the yin aspect
Yin should not be confused with gender, by the way.
Although it is correct to see yin as feminine and yang as masculine, everything in the world is really a mixture of the two, which means that female beings may actually be mostly yang and male beings may actually be mostly yin. Because of that, things that we might expect to be female or male because they clearly represent yin or yang, may turn out to be the opposite instead.