DIBS is a part of the phrase "Calling dibs" and can only be defined in that (rather limited) context.
"Calling dibs" refer to kids at a dinner table. They are NOT allowed to eat before Mum or Dad have said the prayer. But they CAN see the food and decide which food items they want. One kid say "I want a chicken Leg" another say "I want the neck" a third say "I want a piece of breast" by doing that they make it 'bad' for one of the other kids to take the piece the have 'called dibs on'. It works fine as long as there are only a few kids, but when there are many it quickly breaks down and becomes arguments and anger. That can give rise to some pretty complex solutions, like one I read of where a boy grabbed a chicken leg and licked it before putting it back on the serving plate (this to ensure that none of the other kids would now take that leg)
Nowadays a lot of families have abandoned the tradition of saying a prayer before eating ;-))
LyricalOne 