A sexual act where, whilst in the doggystyle position, the male first inserts his index finger in his partner's anus to cover it in feces. Once his finger is sufficiently coated, he then reaches forward to his partner's face and smears it across her (or his) upper forehead, just under the hairline.
Assumed to have originated in China, the Dirty Mao may have been given its name due to the ubiquitous murals of Mao Zedong in which he is depicted with a very advanced receding hairline. The bald area is the target of the Dirty Mao, though an individual with a full head of hair can only receive it just under the hairline that they do have.
Yo dawg, while I was in Beijing, I tried to give this hottie the Dirty Sanchez. But she stopped me and made me put the skid mark on her forehead and told me that it's the Dirty Mao.