In Celtic mythology, a Joint-eater is a type of jinn who sits invisibly and consumes half of their victim's food.
It is also known as an Alp-luachra in Ireland. A man haunted by a joint-eater will never grow fat, because the pith or quintessence of the food is consumed by the jinn. People who consume newts are thought to be plagued in this way.
A folk remedy states that to rid one's self of an Alp-Luachra, one should eat a large quantity of salt beef, without drinking anything, and then lay by a running stream with mouth wide open. After a long wait, the Alp-Luachra will become thirsty, and will jump into the stream to drink.