What is the X in X-mas?
"pertaining to the intestines," 1869, from Greek enterikos "intestinal," first used in this sense by Aristotle, from entera (plural; singular enteron) "intestines," from PIE *enter-, comparative of *en "in" (see inter-).
enteric en·ter·ic (ěn-těr'ĭk)
Of, relating to, or within the intestine.
By way of the intestine; enteral.