c. 1600, "attention to particulars," from French détail, from Old French detail "small piece or quantity," literally "a cutting in pieces," from detaillier "cut in pieces" (12c.), from de- "entirely" (see de-) + taillier "to cut in pieces" (see tailor).
French en détail "piece by piece, item by item" (as opposed to en gros), a commercial term used where we would today use retail, expanded the senses of the noun. Meaning "a minute account or narrative" is from 1690s; that of "an individual part, a particular" is from 1786. In fine arts, "a small, subordinate part," by 1823.
Military sense of "selection of an individual or body of troops for a particular service" is from 1708, from the notion of "distribution in detail of the daily orders first given in general," including assignment of specific duties.
1630s, "relate or narrate in particulars," from French dtailler "cut up in pieces; narrate in particulars," from Old French detaillier "cut in pieces" (12c.), from de- "entirely" (see de-) + taillier "to cut in pieces" (see tailor (n.)). Meaning "divide or set off" (especially for military duty) is from 1793. Related: Detailed; detailing.