c. 1200, "to throw, throw violently, fling, hurl," from a Scandinavian source akin to Old Norse kasta "to throw" (cognate with Swedish kasta, Danish kaste, North Frisian kastin), of uncertain origin. Meaning "to form in a mold" is late 15c. In the sense of "to throw" it replaced Old English weorpan (see warp (v.)), and itself largely has been superseded now by throw, though cast still is used of fishing lines (17c.) and glances (13c.).
From c. 1300 as "emit, give out;" also "throw to the ground;" also "shed or throw off;" also "calculate, find by reckoning; chart (a course)." From late 14c. as "to calculate astrologically." From late 15c. as "bring forth abortively or prematurely." From 1711 as "distribute the parts (of a play) among the actors." Of votes from 1840, American English. To cast up is from 1530s as "compute, reckon," late 15c. as "eject, vomit."
mid-13c., "a throw, an act of throwing," from cast (v.). In early use especially of dice, hence figurative uses relating to fortune or fate. Meaning "that which is cast" is from mid-15c. Meaning "dash or shade of color" is from c. 1600.
The sense of "a throw" carried an idea of "the form the thing takes after it has been thrown," which led to widespread and varied meanings, such as "group of actors in a play" (1630s). OED finds 42 distinct noun meaning and 83 verbal ones, with many sub-definitions. Many of the figurative senses converged in a general meaning "sort, kind, style" (mid-17c.). Meaning "model made from taking an impression of an object" is from c. 1500. A cast in the eye "slight squint" (early 14c.) preserves the older verbal sense of "warp, turn," via the notion of "permanent motion or turn." As "plaster molded around an injured or diseased part," by 1883.