Etymology
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level (n.)

mid-14c., "tool to indicate a horizontal line," from Old French livel "a level" (13c.), ultimately from Latin libella "a balance, level" (also a monetary unit), diminutive of libra "balance, scale, unit of weight" (see Libra). Spanish nivel, Modern French niveau are from the same source but altered by dissimilation.

Meaning "position as marked by a horizontal line" (as in sea-level) is from 1530s; meaning "flat surface" is from 1630s; meaning "level tract of land" is from 1620s. Figurative meaning in reference to social, moral, or intellectual condition is from c. 1600. Figurative phrase on the level "fair, honest" is from 1872; earlier it meant "moderate, without great ambition" (1790).

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caliber (n.)

"inside diameter of a gun barrel," 1580s, from French calibre (by mid-16c., perhaps late 15c.), often said to be ultimately from Arabic qalib "a mold for casting." Barnhart remarks that Spanish calibre, Italian calibro "appear too late to act as intermediate forms" between the Arabic word and the French.

But English Words of Arabic Ancestry finds that the idea of an Arabic source "comes with no evidence and no background historical context to support it. It is far more likely that the word was formed in French" from Medieval Latin qua libra "of what weight" (a theory first published 19c. by Mahn), from fem. ablative of quis (from PIE root *kwo-, stem of relative and interrogative pronouns) + ablative of libra "balance" (see Libra).

In U.S., expressed in decimal parts of an inch (.44-caliber = ".44-inch caliber"). The earliest sense in English is a figurative one, "degree of merit or importance" (1560s), from French. Later, figuratively, "the capacity of one's mind, one's intellectual endowments."

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pound (n.1)

[fundamental unit of weight] Old English pund "pound" (in weight or money), also "pint," from Proto-Germanic *punda- "pound" as a measure of weight (source of Gothic pund, Old High German phunt, German Pfund, Middle Dutch pont, Old Frisian and Old Norse pund), an early borrowing from Latin pondo "pound," originally in libra pondo "a pound by weight," from pondo (adv.) "by weight," ablative of pondus "weight," from stem of pendere "to hang, cause to hang; weigh" (from PIE root *(s)pen- "to draw, stretch, spin"). Perhaps the notion is the weight of a thing measured by how much it stretches a cord.

Meaning "unit of money" was in Old English, originally "a (Tower) pound of silver."

In the Middle Ages it was reckoned variously: the Tower pound (12 ounces), the merchant's pound (15), the avoirdupois (16), the Troy (12); the 16-ounce pound was established before late 14c. Pound cake (1747) is so called because it has a pound, more or less, of each ingredient. Pound of flesh is from "Merchant of Venice" IV.i. The abbreviations lb., £ are from libra "pound," and reflect the medieval custom of keeping accounts in Latin (see Libra).

A peny yn seson spent wille safe a pounde. [Paston Letters, 1457]
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zodiac (n.)
late 14c., from Old French zodiaque, from Latin zodiacus "zodiac," from Greek zodiakos (kyklos) "zodiac (circle)," literally "circle of little animals," from zodiaion, diminutive of zoion "animal" (from PIE root *gwei- "to live").

Libra is not an animal, but it was not a zodiac constellation to the Greeks, who reckoned 11 but counted Scorpio and its claws (including what is now Libra) as a "double constellation." Libra was figured back in by the Romans. In Old English the zodiac was twelf tacna "the twelve signs," and in Middle English also Our Ladye's Waye and the Girdle of the Sky.
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library (n.)
place for books, late 14c., from Anglo-French librarie, Old French librairie, librarie "collection of books; bookseller's shop" (14c.), from Latin librarium "book-case, chest for books," and libraria "a bookseller's shop," in Medieval Latin "a library," noun uses of the neuter and fem., respectively, of librarius "concerning books," from Latin librarium "chest for books," from liber (genitive libri) "book, paper, parchment."

Latin liber (from Proto-Italic *lufro-) was originally "the inner bark of trees," and perhaps is from PIE *lubh-ro- "leaf, rind," a derivative of the PIE root *leub(h)- "to strip, to peel" (see leaf (n.)). Comparing Albanian labë "rind, cork;" Lithuanian luobas "bast," Latvian luobas "peel," Russian lub "bast," de Vaan writes that, "for want of a better alternative, we may surmise that liber is cognate with *lubh- and goes back to a PIE word or a European word 'leaf, rind.'"

The equivalent word in most Romance languages survives only in the sense "bookseller's shop" (French libraire, Italian libraria). Old English had bochord, literally "book hoard." As an adjective, Blount (1656) has librarious.
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librarian (n.)
"custodian of a library," 1713; see library + -an. Earlier form was library-keeper (1640s), and librarian had been used in the sense "scribe, one who copies books" (1660s).
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scale (n.2)

[weighing instrument] early 15c., extended to the whole instrument from the earlier sense of "pan of a balance" (late 14c.); earlier still "drinking cup" (c. 1200), from Old Norse skal "bowl, drinking cup," in plural, "weighing scale."

This is from a noun derivative of Proto-Germanic *skæla "to split, divide" (source also of Old Norse skel "shell," Old English scealu, Old Saxon skala "a bowl (to drink from)," Old High German scala, German Schale "a bowl, dish, cup," Middle Dutch scale, Dutch schaal "drinking cup, bowl, shell, scale of a balance"), from PIE root *skel- (1) "to cut."

The connecting sense seems to be of half of a bivalve ("split") shell used as a drinking cup or a pan for weighing; compare scallop, which is from the same root. But according to Paulus Diaconus the "drinking cup" sense originated from a supposed custom of making goblets from skulls (see skull). Scales as a name for the zodiac constellation Libra is attested in English from 1630s.

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balance (n.)

early 13c., "scales, apparatus for weighing by comparison of mass," from Old French balance "balance, scales for weighing" (12c.), also in figurative sense; from Medieval Latin bilancia, from Late Latin bilanx, from Latin (libra) bilanx "(scale) having two pans," possibly from Latin bis "twice" (from PIE root *dwo- "two") + lanx "dish, plate, scale of a balance," which is of uncertain origin.

The accounting sense "arithmetical difference between the two sides of an account" is from 1580s; meaning "sum necessary to balance the two sides of an account" is from 1620s. Meaning "what remains or is left over" is by 1788, originally in commercial slang. Sense of "physical equipoise" is from 1660s; the meaning "general harmony between parts" is from 1732.

Many figurative uses are from Middle English image of the scales in the hands of personified Justice, Fortune, Fate, etc.; thus in (the) balance "at risk, in jeopardy or danger" (c. 1300). Balance of power in the geopolitical sense "distribution of forces among nations so that one may not dominate another" is from 1701. Balance of trade "difference between the value of exports from a country and the value of imports into it" is from 1660s.

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