c.1300, "entrance, arrival," lit. "what enters," perhaps a noun use of the late O.E. verb incuman "come in," from in (adv.) + cuman "to come" (see come). Meaning "money made through business or labor" first recorded 1601. 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. Incoming was originally of game approaching the hunter.