1. A penis long enough that it can penetrate both the vagina and the cervix.
2. In anal sex, a penis long enough that it can penetrate both the Anus and the Houston Valves.
Note: If you're a dumb shit who can't figure out what the Houston Valves are, please consult Google.
Black Guy: Double Docker
White Guy: Docker ... hits the G-spot
Asian Guy: No Docker ... she can't even feel it inside her.