The name given by Kirk to what the Irish call "Alp-Luachra"; but, according to Kirk, this Joint-eater is a kind of fairy who sits invisibly beside his victim and shares his food with him:

They avouch that a Heluo, or Great-eater, hath a voracious Elve to be his attender, called a Joint-eater or Just-halver, feeding on the Pith or Quintessence of what the Man eats; and that therefoir he continues Lean like a Hawke or Heron, notwithstanding his devouring Appetite.

"The Secret Commonwealth of Elves, Fauns, and Fairies" by Robert Kirk (1291: p.71)

In Ireland this phenomenon is accounted for by the man having swallowed a newt when sleeping outside by a running stream. In Douglas hyde's Beside the Fire, there is a detailed account of a man infested by a pregnant Alp-Luachra, and the method by which he was cleared of the thirteen Alp-Luachra by Mac Dermott the Prince of Coolavin. In all the stories the method is the same: the patient is forced to eat a great quantity of salt beef without drinking anything, and is made to lie down with his mouth open above a stream, and after a long wait the Alp-Luachra will come out and jump into the stream to quench their thirst. But this is folk-medicine, not fairy-lore; it is Kirk who attributes the unnatural hunger to an Elf (1290: p.243).