A dancestor is a term coined by Porrim Maryam in the of Openbound, and later reiterated by Horuss Zahhak in the , that refers to a player's ancestor on one side of the Scratch who, due to changes caused by the Scratch, is a descendant on the other side.
Due to the complexities in familial relationships from each player group considering themselves the descendants and their counterpart player group the ancestors, causing confusing terminology, the term originated as a portmanteau of descendant and ancestor as a single simple term to describe the relationship. It is likely that Porrim was referring primarily to troll players, possibly as a reference to the fact that it is not apparently known which set is the biological ancestry of the other.
By contrast, for the humans, it is known that the B2 kids are the parents of the B1 kids. However, as the term appears to mostly deal with the "perceived" direction of the relationships, it could be seen as applicable to the humans as well; as few of them are aware that their grandparents/ancestors are their children. Roxy started to catch on to it after seeing Rose in a dream bubble for the second time. Jade had long since been aware that her "pen pal" is her grandson, but she had never indicated awareness that Jake is also her grandfather. As of the Epilogues, all the human kids know who was who in each instance of Universe B.
While the term's origin has nothing to do with dancing, the obvious potential for puns was highlighted by Horuss when he mentioned the term; he provides a horse pun version, with the hashtag "dressagecestor". Act 6 Intermission 3 also happens to be titled "Ballet of the Dancestors".