1580s, "Oriental council of state," from Turkish divan, from Arabic diwan, from Persian devan "bundle of written sheets, small book, collection of poems" (as in the "Divan i-Hafiz"), related to debir "writer."
Sense evolved in Arabic through "book of accounts," to "office of accounts," "custom house," "council chamber," then to "long, cushioned seat," such as are found along the walls in Middle Eastern council chambers. The sofa/couch sense was taken into English 1702; the "book of poems" sense (especially a collection by a single author) by 1787.
c. 1300, "a throwing," verbal noun from cast (v.). From early 15c. as "the casting of metal, the act or process of founding;" 1788 as "a metal casting, that which has been formed by running molten metal into a mold of a desired form." Theatrical sense is from 1814. Casting couch "divan in a Hollywood casting directors office," with suggestion of sexual favors in exchange for a role in a picture is by 1948.
mid-14c., "a bed," from Old French couche "a bed, lair" (12c.), from coucher "to lie down," from Latin collocare (see couch (v.)). From mid-15c. as "a long seat upon which one rests at full length." Traditionally, a couch has the head end only raised, and only half a back; a sofa has both ends raised and a full back; a settee is like a sofa but may be without arms; an ottoman has neither back nor arms, nor has a divan, the distinctive feature of which is that it goes against a wall.
As symbolic of a psychiatric treatment or psychoanalysis, by 1952. Couch potato first recorded 1979.