1806, "pertaining to a division of a larger part;" see section (n.) + -al (1). Originally and especially "of or pertaining to some particular section or region of a country as distinct from others," in which sense it loomed large in the U.S. political vocabulary in the decades before the Civil War.
The meaning "composed or made up of several independent sections that fit together" is by 1875, originally mechanical. The noun meaning "piece of furniture composed of sections which can be used separately" is attested by 1961, short for sectional seat, sectional sofa, etc. (1949).
updated on March 25, 2022