The first person to bagsy something, such as an item or activity, becomes the undisputed owner/participant. For example, saying "bagsy the biggest cake" would give you the right to the biggest cake, or "bagsy not doing the washing up", would give you the right not to do the washing up. If there are more than two people involved, it is usually the last to bagsy that has no choice in the matter, or has to do the unpleasant task.
To shotgun something is a variation of to bagsy, but can only be applied to objects within sight. This undisputed right to use the object only lasts until it goes out of sight again. A very common example of this is to shotgun the front seat of a vehicle.
