Why was "tantrum" trending last week?
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.
a regiment of the Roman army, the number of men composing which differed at different times. It originally consisted of three thousand men, but in the time of Christ consisted of six thousand, exclusive of horsemen, who were in number a tenth of the foot-men. The word is used (Matt. 26:53; Mark 5:9) to express simply a great multitude.