Etymology
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Dewey Decimal system (n.)

library classification system that organizes information into 10 broad areas subdivided numerically into progressively smaller topics, by 1885, named for Melvil Dewey (1851-1931) who proposed it 1876 while acting librarian of Amherst College. He also crusaded for simplified spelling and the metric system.

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cardinal number (n.)
1590s, "one, two, three," etc. as opposed to ordinal numbers "first, second, third," etc.; so called because they are the principal numbers and the ordinals depend on them (see cardinal (adj.)).
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Morse code (n.)

character encoding system originally invented for use with the telegraph, by 1860, earlier Morse key (1858), so called in honor of Samuel F.B. Morse (1791-1872), U.S. inventor who produced a system of telegraphic communication in 1836. He invented both the recording telegraph and the alphabet of dots and dashes.

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feng shui (n.)
also feng-shui, fung-shui, 1797, from Chinese, from feng "wind" + shui "water." A system of spiritual influences in natural landscapes and a means of regulating them; "A kind of geomancy practiced by the Chinese for determining the luckiness or unluckiness of sites for graves, houses, cities, etc." [Century Dictionary].
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rank and file (n.)

1590s, in reference to the horizontal and vertical lines of soldiers marching in formation, from rank (n.) in the military sense of "number of soldiers drawn up in a line abreast" (1570s) + file (n.1). Thence generalized to "common soldiers" (1796) and "common people, general body" of any group (1860).

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kit and caboodle (n.)
also kaboodle, 1870, earlier kit and boodle (1855), kit and cargo (1848), according to OED from kit (n.1) in dismissive sense "number of things viewed as a whole" (1785) + boodle "lot, collection," perhaps from Dutch boedel "property." Century Dictionary compares the whole kit, of persons, "every one" (1785).
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seven-year itch (n.)
1899, American English, some sort of skin condition (sometimes identified with poison ivy infection) that either lasts seven years or returns every seven years. Jocular use for "urge to stray from marital fidelity" is attested from 1952, as the title of the Broadway play (made into a film, 1955) by George Axelrod (1922-2003), in which the lead male character reads an article describing the high number of men have extra-marital affairs after seven years of marriage.
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primum mobile (n.)

"the first source of motion," mid-15c., from Medieval Latin (11c.), literally "the first movable thing;" see prime (adj.) + mobile.

In the Ptolemaic system of astronomy, the tenth or outermost of the revolving spheres of the universe, which was supposed to revolve from east to west in twenty-four hours, and to carry the others along with it in its motion; hence, any great or first source of motion. [Century Dictionary]

A translation of Arabic al-muharrik al-awwal "the first moving" (Avicenna). Englished by Chaucer as the firste Moeuyng (c. 1400). Old science also had primum frigidum "pure cold: an elementary substance, according to the doctrine of Parmenides."


 

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a cappella 

1868, earlier alla capella (1824), from Italian, "in the style of Church music, in the manner of the chapel," literally "according to the chapel," from cappella "chapel" (see chapel). Originally in reference to older church music (pre-1600) which was written for unaccompanied voices; applied 20c. to unaccompanied vocal music generally. Italian a is from Latin ad "to, toward; for; according to" (see ad-); alla is a la "to the." Sometimes in the Latin form a capella.

Also denoting "that instruments are to play in unison with the voices, or that one part is to be played by a number instruments." ["Chambers's' Encyclopaedia," 1868]

You are not the first person puzzled by the expression "A Capella," or, at any rate, unable to understand it should signify the exact reverse of what it literally does signify. The chorales in oratorios were invariably accompanied, either by double-bass or the whole band. Hence they were, with perfect correctness, said to be performed "a capella." But, as other chorales, sung as part of the church service, were written in the same and simple style the expression "a capella" came in time to be applied to them also, despite their being sung without any instrumental accompaniment whatever. [The Music World, Sept. 11, 1875]
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