I spotted an interesting observation about Rephaim from JoAnn Scurlock, in “Mortal and Immortal Souls, Ghosts and the (Restless) Dead in Ancient Mesopotamia”, Religion Compass 10, no. 4 (2016): 77–82 (79). She is discussing how Ancient Mesopotamians treated the dead.
Having a family tomb under the floor of the house made funerary offerings by the family as a group a simple matter as long as the family survived or new owners of the house continued to use the tomb. What would happen then is that, as the memory of the deceased faded and the bones of the long dead mingled with those of more recent arrivals, the individual eøemmu’s [‘ghosts’] melded into a common eøem kimti (Scurlock 2013, pp. 151–152). Eventually, this collective ancestor mixed with the wider community of the long dead, the kimtu rapaåtu, literally ‘widespread relations’. Of interest to Biblical scholars puzzled by the term rephaim is the fact that an old Babylonian commentary (5R 44: 121 [sic]) uses the term kimtu rapaåtu to translate Amorite rapi (singular of rephaim). This would seem to indicate that the mysterious Rephaim are the ghosts of persons who have been dead for a very long time.
“5R 44” (or “VR 44”) is a so-called Name Book from Ashurbanipal’s library (Ashurbanipal was an Assyrian king who reigned 668-627 BC). The text provides a list of Akkadian translations of non-Akkadian names. The reference is to column 1 line 21, so there should have been a gap in the cited reference followed by a Roman numeral: 5R 44: I 21.
5R 44: I 21 reads mḪa-am-mu-ra-pí : mKim-ta-ra-pa-áš-tum, the meaning of each name being “great family” or as CAD K has it (p. 377, s.v. kimtu), “extensive family”. The “ra-pi” means “great/extensive”, and ‘Ammu means “family”. So “rapi” itself does not refer to the long dead.
In the Bible, the Rephaim are either peoples discovered as inhabiting Canaan and neighbouring territories when the Israelites invade (so are long dead from the perspective of the writers) or, in poetic and prophetic books, are long-dead inhabitants of the netherworld. In 5R 44, they are also described as “kings”, another feature in common with many biblical Rephaim, and more consistent with the meaning of “great”.