The aroma left upon one's fingers after insertion into a woman's vagina during sex; implies lack of proper hygene, not on the part of the person achieving "stink finger," but on the part of the woman involved.
I had sex with Vicky last night and left with an amazinglypungent case of stink finger.
Sticking your finger(s) in or around your anus, just enough to get a poop smell on it. Then, putting your "stink finger" up to someone else's nose, forcing them to smell the poop.
My wife came up to me when I was lying on the couch, and farted in my face. I ran after her, and gave her the stink finger.