To put “dibs” on something, like a piece of cake, or Dad’s car, is to express first claim to a share of that object.
The word dates from before 1700 and comes from a very old children’s game called dibstone.
The game was a forerunner to marbles, which was played with either sheep knuckles or small stones.
The object was to capture an opponent’s stone by declaring, “I dibs!” meaning, “It’s mine!”
The plural of the word was dubs.