A) The Lucky Cinder-Block is a fictitious object carried by backpackers and students. It represents any extremely heavy object carried for no particular reason.
B) Any useless, weighty, or otherwise pointless object occupying an inordinate amount of space in a backpack.
Steve: Hey Joe, Why is your backpack so heavy?
Joe: I don't know.
Steve: Dude, you brought your lucky cinder-block along didn't you?