also lockup, "detention cell for offenders," 1838, perhaps short for earlier lock-up house; from the verbal phrase. Meaning "action of locking up" is from 1845. The verbal phrase lock (someone) up in a dwelling, prison, etc., is from early 15c. Of things, "to hold in safekeeping or concealment," also early 15c. See lock (v.) + up (adv.). To lock up (intransitive) "lock all the doors" (of a house, shop, etc.) is from 1901.