late 14c., "action of coming in," from incoming (adj.), which is attested from 1753. As "that which is coming in" from 1892, originally of game; transferred in World War I to artillery; as a warning cry of incoming shellfire, it seems to date to the U.S. war in Vietnam (1968).

1753, "coming in as an occupant," present-participle adjective from in (adv.) + come (v.). Of game, from 1892; transferred in World War I to artillery; as a warning cry of incoming shellfire, it seems to date to the U.S. war in Vietnam (1968).

"group of experts assembled to give advice on some matter," occasionally used since early 1900s, it became current in 1933, in reference to the intellectuals gathered by the administration of incoming U.S. President Franklin D. Roosevelt as advisers; from brain (n.) + trust (n.).