when someone copulates with their own bloodrelatives. In the states it's relatively sick. It's when some dude bangs their own relative.
Some time in the early 2000s, a mother and a son appeared on The Jerry Springer Show saying they were in love with each other, but in truth they were committing the sick act of incest. They wanted to have a baby together, and the crowd chanted "three headed baby, three headed baby, three headed baby."
The conclusion of that one was that the mother and son are motherfucking freaks. Who'd fuck their own mother? Can you imagine what their fuck child would look like -- *shudder*
It's like when a brother and sister are bumping uglies then the sister gets knocked up. They don't know what the fuck to call their kid (son/nephew or mother/aunt)