The Gemara relates the case of a bustana – an orchard (the source of the word is in the Persian bostan which literally mean “the place of the wind,” but was used even in the original to mean an orchard) – that bordered on the wall of an apadna (in ancient Persian an appadana meant “the king’s palace.” The term was borrowed by the Aramaic language, where it appears in the book of Daniel to mean a palace or a fancy dwelling). The orchard was thus considered a karpef she-mukaf ledira, since it shared one wall with the palace.
In this case, the outer wall between the orchard and the palace collapsed. Rav Beivai suggested that carrying could still be permitted in the orchard, relying on the inner wall of the palace. Rav Pappi rejects that logic, arguing that although the original wall was built to service both the palace and the orchard, the remaining wall was built only for the palace, and not for the orchard. Therefore he rules that the orchard has lost its status as a karpef she-mukaf ledira and carrying in it will now be forbidden.
In rejecting Rav Beivai’s argument, Rav Pappi gently mocks him by saying “because you come from mula’ei people you speak mulayata matters.” Rashi interprets this expression as referring to Rav Beivai’s family history. Rav Beivai was Abaye’s son, and Abaye was from the family of Eli ha-Kohen (see Rosh ha-Shana 18a), whose family had a tradition of dying at a young age, because of the curse invoked against them (see I Samuel 3:10-14). The term is understood to mean “truncated” and in this case means “because you come from a family that is truncated (cut off) before they reach old age, you offer suggestions that are truncated” – i.e. have no support to them.
The Rashbam accepts that the expression stems from Rav Beivai’s family tree, but argues that Mula’ei is the name of the place that Eli ha-Kohen’s sons lived.
According to the Ge’onim this is simply an expression that was used when responding to an important person whose suggestion appears to be rash.