A rum runner is a type of flask used to sneak alcohol into a social place that prohibits bringing any. For example many people flasks like this to sneak alcohol onto cruise ships but can be used anywhere.
Craig: Dude I lost forgot my wallet I can't buy any drink.
Josh: don't worry I got my rum runner
Someone who is extremely stupid. They will often lack even the basic knowledge common to small children. Derived from the derogatory phrase "the best part of you ran down your mother's leg."
Person 1: "Yo, that guy just asked me what part of AfricaCanada was in!"