legion (n.)
c.1200, from Old French legion "Roman legion" (3,000 to 6,000 men, under Marius usually with attached cavalry), from Latin legionem (nominative legio) "body of soldiers," from legere "to choose, gather," also "to read" (see lecture (n.)).

Generalized sense of "a large number" is due to translations of allusive phrase in Mark v:9. American Legion, U.S. association of ex-servicemen, founded in 1919. Legion of Honor is French légion d'honneur, an order of distinction founded by Napoleon in 1802. Foreign Legion is French légion étrangère "body of foreign volunteers in a modern army," originally Polish, Belgian, etc. units in French army; they traditionally served in colonies or distant expeditions.