What is the X in X-mas?
late 14c., possibly a shortened variant of stricklen "to trickle," a frequentative form of striken "to flow, move" (see strike). Related: Trickled; trickling. Trickle-down as an adjectival phrase in an economic sense first recorded 1944; the image had been in use at least since Teddy Roosevelt.
1570s, from trickle (v.).