The heffelfinger is a well used sex term. When the male is doing his partner from behind, preferrably doggy style, he will stick his index finger in her rectum. When there is a change of position, preferrably the male going on top, he will put the index finger into his partner's mouth.
When she asks if that was the finger in her rectum, the male replies, "Yes, thats the heffelfinger."
by Convicted Heffelfingerer April 30, 2007