What is the X in X-mas?
early 14c., "become greater in size or number; to cause to grow, enlarge," from Anglo-French encress-, Old French encreiss-, present participle stem of encreistre, from Latin increscere "to increase, to grow upon, grow over, swell, grow into," from in- "in" (see in- (2)) + crescere "to grow" (see crescent). Latin spelling restored 15c. Related: Increased; increasing.
late 14c., "action of increasing; results of an increasing," from increase (v.).