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Old English cealc "chalk, lime, plaster; pebble," a West Germanic borrowing from Latin calx (2) "limestone, lime (crushed limestone), small stone," from Greek khalix "small pebble," which many trace to a PIE root for "split, break up." In most Germanic languages still with the "limestone" sense, but in English transferred to the opaque, white, soft limestone found abundantly in the south of the island. Modern spelling is from early 14c. The Latin word for "chalk" was creta, which also is of unknown origin.
1570s, "to mix with chalk;" 1590s as "to mark with chalk," from chalk (n.). Related: Chalked; chalking. Old English had cealcian "to whiten." Certain chalk marks on shipped objects meant "admitted" or "shipped free," hence some figurative senses. Chalk boards also were commonly used in keeping credit, score, etc., hence figurative use of chalk it up (1903).
A soft, white, gray, or yellow limestone consisting mainly of calcium carbonate and formed primarily from the accumulation of fossil microorganisms such as foraminifera and calcareous algae. Chalk is used in making lime, cement, and fertilizers, and as a whitening pigment in ceramics, paints, and cosmetics. The chalk used in classrooms is usually artificial.
A horse favored to win
[1950s+ Horse racing; References to winning by a long chalk, an allusion to scoring points by a chalk mark, date from the 1830s]