calculus n.

Old English cealc "chalk, soft white limestone; lime, plaster; pebble," a West Germanic borrowing from Latin calx (2) "limestone, lime (crushed limestone), small stone," borrowed from Greek khalix "small pebble," which many trace to a PIE root for "split, break up," but Beekes writes that "There is no convincing etymology."

Cognate words in most Germanic languages still have the "limestone" sense, but in English transferred chalk to the opaque, white, soft limestone found abundantly in the south of the island. The modern spelling is from early 14c. The Latin word for "chalk" was creta, which also is of unknown origin. With many figurative or extended senses due to the use of chalk marks to keep tracks of credit for drinks in taverns and taprooms, or to keep the score in games.

1560s, "ascertain by computation, estimate by mathematical means," from Latin calculatus, past participle of calculare "to reckon, compute," from calculus (see calculus). It replaced earlier calculen (mid-14c.), from Old French calculer.

The meaning "to plan, devise" is attested from 1650s; hence "to purpose, intend" and "think, guess" (1830), both U.S. idioms. Related: Calculable.

late 14c., "mathematician, one who calculates," from Latin calculator, from calculatus, past participle of calculare "to reckon, compute," from calculus "reckoning, account" (see calculus). In reference to mechanical adding machine contraptions from 1784; of electronic ones from 1946.

Electronic calculator uses 18,000 tubes to solve complex problems [Scientific American headline, June 1946]