Why is the ninth month called September?
c.1600, from French détail, from Old French detail "small piece or quantity," literally "a cutting in pieces," from detaillier "cut in pieces," from de- "entirely" (see de-) + taillier "to cut in pieces" (see tailor).
Modern sense is from French en détail "piece by piece, item by item" (as opposed to en gros), a commercial term used where we would today use retail. Military sense is 1708, from notion of "distribution in detail of the daily orders first given in general," including assignment of specific duties.
1630s, from French détailler "cut up in pieces; narrate in particulars," from Old French detaillier, from detail (see detail (n.)). Related: Detailed; detailing.