"a head-rest used by a person reclining," especially a soft, elastic cushion filled with down, feathers, etc., Middle English pilwe, from Old English pyle "cushion, bed-cushion, pillow," from West Germanic *pulwi(n) (source also of Old Saxon puli, Middle Dutch polu, Dutch peluw, Old High German pfuliwi, German Pfühl), an early borrowing (2c. or 3c.) from Latin pulvinus "little cushion, small pillow," of uncertain origin. The modern spelling in English is from mid-15c.
Pillow fight (n.) "mock combat using pillows as weapons" is attested from 1837; slang pillow talk (n.) "relaxed intimate conversation between a couple in bed" is recorded by 1939. Pillow-case "washable cloth drawn over a pillow" is by 1745. Pillow-sham is by 1867.