HCDSC

Stands for 'Human Centipede Dick-Sucking Chain'

Person one sucks person two's dick, person two sucks person three's dick, and so on...

In the end the last person might suck the first person's dick, making it a circle/ring, but that is not necessary.

Person 1: Do you know what the HCDSC is?
Person 2: No, what is that?
Person 1: It stands for 'Human Centipede Dick-Sucking Chain', you want in?
Person 2: Yes! Of course!