"small case, natural or artificial," 1650s, from French capsule "a membranous sac" (16c.), from Latin capsula "small box or chest," diminutive of capsa "box, case, chest" (see case (n.2)). Medicinal sense is 1875; shortened form cap is from 1942. Sense in space capsule is first recorded 1954, perhaps from earlier sense "shell of a metallic cartridge" (1864). As an adjective from 1938. Related: Capsular.