c. 1300, curtine, "hanging screen of textile fabric used to close an opening or shut out light, enclose a bed, or decorate an altar," from Old French cortine "curtain, tapestry, drape, blanket," from Late Latin cortina "curtain," but in classical Latin "round vessel, cauldron," from Latin cortem (older cohortem) "enclosure, courtyard" (see cohort).
The meaning shift apparently begins with cortina being used as a loan-translation of Greek aulaia ("curtain") in the Vulgate (to render Hebrew yeriah in Exodus xxvi:1, etc.). The Greek word was connected to aule "court," perhaps because the "door" that led out to the courtyard of a Greek house was a hung cloth.
Figuratively from early 15c. as "something that conceals or screens." From 1590s as "large sheet used to conceal the stage in a theater." Many of the figurative senses are from stage plays: Behind the curtain "concealed" is from 1670s; curtains "the end" is by 1912. The theatrical curtain call "appearance of individual performers on stage at the end of a performance to be recognized by the audience" is from 1884. To draw the curtain is from c. 1500 in opposite senses: "to conceal," and "to reveal." Curtain-rod is from c. 1500. An Old English word for "curtain" was fleonet "fly-net."