A comment intended to display knowledge of a subject which instead exposes the ignorance of the person who made it. Derived from a scene in "The 40 Year Old Virgin" when the main character's virginity is obvious due to his story about a woman's breast feeling like a bag of sand.
Word used to describe a lady you'd like to analy intrude with absolutely no lube whatsoever. So dry would you like to drybum her that you intentionally take a "bag of sand" out with you on a night out.