dibs (interj.)

children's word to express a claim on something, 1915, originally U.S., apparently from earlier senses "a portion or share" and "money" (early 19c. colloquial), probably a contraction of dibstone "a knuckle-bone or jack in a children's game" (1690s), in which the first element is of unknown origin. The game consisted of tossing up small pebbles or the knuckle-bones of a sheep and catching them alternately with the palm and the back of the hand.

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Definitions of dibs from WordNet

dibs (n.)
a claim of rights;
I have dibs on that last slice of pizza