What is the X in X-mas?
c.1300, "entrance, arrival," literally "what enters," perhaps a noun use of the late Old English verb incuman "come in," from in (adv.) + cuman "to come" (see come). Meaning "money made through business or labor" (i.e., "that which 'comes in' as a product of work or business") first recorded c.1600. Income tax is from 1799, first introduced in Britain as a war tax, re-introduced 1842; authorized on a national level in U.S. in 1913.