What is the X in X-mas?
"the hills of the fairies," 1793; but in Yeats, "the fairie folk" (1899), elipsis of Irish (aos) sidhe "people of the faerie mound" (cf. second element in banshee).
in Irish folklore, a hill or mound under which fairies live. The phrase aos sidhe or the plural sidhe on its own (sometimes anglicized as shee) can denote fairy folk collectively. See also banshee.