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.
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.
"possessing discernment, noting distinctions and differences accurately and in detail," 1792, present-participle adjective from discriminate (v.).
c. 1500, "in a serious manner, with serious thought or application," from serious (adj.) + -ly (2). By 1765 as "to a serious extent, so as to give grounds for alarm." To take (something) seriously "be earnest in one's attitude toward" is from 1782.
Middle English also had seriousli "minutely, in detail" (late 14c., now obsolete), from Medieval Latin seriose "item by item, in detail," which reflects the confusion with the stem of series, for which confusion see serious (adj.). Caxton (mid-15c.) uses seriately for "attentively, earnestly."