What is the X in X-mas?
1620s, from French déférent (16c.), from Latin deferentem (nominative deferens), present participle of deferre "to carry down or away" (see defer (v.2)). Earlier in Middle English as a word in astronomy (early 15c.).
deferent def·er·ent (děf'ər-ənt, děf'rənt)
Carrying down or away, as a duct or vessel.