The other definition here claiming that yin yang refers to anus is correct, and older people in certain regions of the US know this very well. This site is for "slang" usages, not the formal dictionary definition which would refer to the Chinese symbol. When a person says something like, "Wow, this store has widgets up the yin yang", it does not mean plethora. Rather, it's a substitution for the word ass. And in a sexual context, yin yang does indeed refer to the anus. "Taking it up the yin yang" is probably the most used phrase, although it's now somewhat antiquated, and it means taking it up the anus, or male/female part which is common to men and women, which is how the word was derived since the Chinese symbol represents the male/female balance.
The drug smuggler stuffed it up his yin yang.