Etymology
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fifth column (n.)
1936, from Gen. Emilio Mola's comment at the siege of Madrid during the Spanish Civil War that he would take the city with his four columns of troops outside it and his "fifth column" (quinta columna) in the city.
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column (n.)

mid-15c., "a pillar, long, cylindrical architectural support," also "vertical division of a page," from Old French colombe (12c., Modern French colonne "column, pillar"), from Latin columna "pillar," collateral form of columen "top, summit," from PIE root *kel- (2) "to be prominent; hill."

In the military sense "formation of troops narrow in front and extending back" from 1670s, opposed to a line, which is extended in front and thin in depth. Sense of "matter written for a newspaper" (the contents of a column of type) dates from 1785.

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fifth (adj., n.)

"next in order after the fourth; an ordinal numeral; being one of five equal parts into which a whole is regarded as divided;" c. 1200, fift, from Old English fifta "fifth," from Proto-Germanic *finftan- (source also of Old Frisian fifta, Old Saxon fifto, Old Norse fimmti, Dutch vijfde, Old High German fimfto, German fünfte, Gothic fimfta),from *fimfe "five" (from PIE root *penkwe- "five") + *-tha, the word-forming element used to make ordinal numbers (see -th (1)). Normal development would have yielded fift; altered 14c. by influence of fourth.

Noun meaning "fifth part of a gallon of liquor" is first recorded 1938, American English; the noun in the music sense is from 1590s. Fifth Avenue (in New York City) has been used figuratively for "elegance, taste" at least since 1858. Fifth wheel "superfluous person or thing" attested from 1630s. It also was the name of a useful device, "wheel-plate or circle iron of a carriage" placed on the forward axle for support and to facilitate turning (1825). And the phrase sometimes is turned on its head and given a positive sense of "that which a prudent driver ought to take with him in case one of the others should break" (1817). Fifth-monarchy-man, 17c. for "anarchist zealot," is a reference to Daniel ii.44.

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columnar (adj.)

"having the form of a column; of or pertaining to a column," 1728, from Late Latin columnaris "rising in the form of a pillar," from columna "column" (see column).

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quintillion (n.)
1670s, from Latin quintus "the fifth" (from PIE root *penkwe- "five") + ending from million. Compare billion. In Great Britain, the fifth power of a million (1 followed by 30 zeroes); in U.S. the sixth power of a thousand (1 followed by 18 zeroes).
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quintet (n.)

1811, also quintette, "composition for five solo voices or instruments," from Italian quintetto, diminutive of quinto "fifth," from Latin quintus "the fifth," related to quinque "five" (from PIE root *penkwe- "five"). Earlier in English in the Italian form, quintetto (1792). Meaning "a company of five singers or players" is from 1882.

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stria (n.)
plural striae, "narrow stripe, groove," 1560s, from Latin stria "a furrow, flute of a column" (see striate).
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at-bat (n.)
"baseball player's turn at the plate," 1912, originally a column heading in statistics tables, from the prepositional phrase.
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backbone (n.)
"spine, vertebral column," early 14c., from back (n.) + bone (n.). Figurative sense of "firmness of purpose, strength of character" is by 1843.
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capital (n.3)
"head of a column or pillar," late 13c., from Anglo-French capitel, Old French chapitel (Modern French chapiteau), or directly from Latin capitellum "head of a column or pillar," literally "little head," diminutive of caput "head" (from PIE root *kaput- "head").
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