Etymology
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fifteen (adj., n.)

"1 more than fourteen; the number which is one more than fourteen; a symbol representing this number;" Old English fiftyne, from fif "five" (from PIE root *penkwe- "five") + tyne (see -teen). For vowel shift, see met (v.). Cognate with Old Saxon fiftein, Old Frisian fiftine, Old Norse fimtan, Swedish femton, Dutch vijftien, German fünfzehn, Gothic fimftaihun "fifteen." French quinze, Italian quindici "fifteen" are from Latin quindecim (from quinque "five;" see quinque-; + -decim (see -teen). The number of players forming a side in rugby.

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fifteenth (adj., n.)

"next in order after the fourteenth; an ordinal numeral; being one of fifteen equal parts into which a whole is regarded as divided;" late 14c., from fifteen + -th (1). By 15c. displacing forms derived from Old English fifteoða. Compare Old Frisian fiftuda, Dutch vijftiende, German fünfzehnte, Old Norse fimmtandi, Gothic fimftataihunda, with ordinal -d where English has -th. As a noun by late 14c.

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quinzane (n.)

also quinzaine, "group of fifteen; period of fifteen days," 1855, in a historical context, from French quinzaine "the number fifteen; a fortnight" (12c.), from quinze "fifteen," from Latin quindecim (see fifteen). In older use, "the fourteenth day after a feast day" (the fifteenth day by inclusive reckoning), mid-15c. French quinze also was the English name of a popular 18th century card game in which the winner was the first to get 15 points or closest to it without going over.

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*penkwe- 

Proto-Indo-European root meaning "five."

It forms all or part of: cinquain; cinque; cinquecento; cinquefoil; fifteen; fifth; fifty; fin (n.) "five-dollar bill;" finger; fist; five; foist; keno; parcheesi; penta-; pentacle; pentad; Pentateuch; Pentecost; pentagon; pentagram; pentameter; pentathlon; Pentothal; Pompeii; Punjab; punch (n.2) "type of mixed drink;" quinary; quincunx; quinella; quinque-; quinquennial; quint; quintain; quintet; quintile; quintessence; quintillion; quintuple.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit panca, Greek pente, Latin quinque, Old Church Slavonic pęti, Lithuanian penki, Old Welsh pimp, Old English fif, Dutch vijf, Old High German funf.

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*dekm- 

Proto-Indo-European root meaning "ten."

It forms all or part of: cent; centenarian; centenary; centi-; centime; centurion; century; centennial; cinquecento; dean; deca-; decade; decagon; Decalogue; Decameron; decapod; decathlon; December; decennial; deci-; decile; decimal; decimate; decimation; decuple; decussate; denarius; denier (n.) "French coin;" dicker; dime; dinar; doyen; dozen; duodecimal; duodecimo; eighteen; fifteen; fourteen; hecatomb; hendeca-; hundred; icosahedron; nineteen; nonagenarian; octogenarian; Pentecost; percent; quattrocento; Septuagint; sexagenarian; seventeen; sixteen; ten; tenth; thirteen; thousand; tithe.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dasa, Avestan dasa, Armenian tasn, Greek deka, Latin decem (source of Spanish diez, French dix), Old Church Slavonic deseti, Lithuanian dešimt, Old Irish deich, Breton dek, Welsh deg, Albanian djetu, Old English ten, Old High German zehan, Gothic taihun "ten."

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met (v.)
past tense and past participle of meet (v.). Old English long vowels tended to shorten before many consonant clusters. Hence meet/met (earlier mette), five/fifteen, house/husband, break/breakfast.
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sixteen (adj., n.)

"1 more than fifteen, twice eight; the number which is one more than fifteen; a symbol representing this number;" Old English sixtyne, from siex (see six) + -teen. Similar formation in Old Frisian sextine, Middle Dutch sestien, Dutch zestien, German sechzehn, Old Norse sextan.

The age of the gods is always sixteen. Sixteen represents the number of perfection, of plenitude. In man it is after the sixteenth year that the first elements of decay begin to appear, and when the moon reaches the sixteenth digit it begins to decrease. [Alain Daniélou, "The Myths and Gods of India"]

From Latin contracted form sexdecim, sedecim come Italian sedici, French seize.

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indiction (n.)
late 14c., "period of fifteen years," a chronological unit of the Roman calendar that continued in use through the Middle Ages, from Latin indictionem (nominative indictio), literally "declaration, appointment," noun of action from past participle stem of indicere "to declare publicly, proclaim, announce," from in- "in" (from PIE root *en "in") + dicere "to speak, say, tell" (from PIE root *deik- "to show," also "pronounce solemnly").

Fixed by Constantine and reckoned from Sept. 1, 312. Originally for taxation purposes, it was "a common and convenient means for dating ordinary transactions" [Century Dictionary]. The name refers to the "proclamation," at the beginning of each period, of the valuation upon which real property would be taxed.
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chaperon (n.)

"woman accompanying and guiding a younger, unmarried lady in public," 1720, from French chaperon "protector," especially "female companion to a young woman," earlier "head covering, hood" (c. 1400), from Old French chaperon "hood, cowl" (12c.), diminutive of chape "cape" (see cap (n.)). "... English writers often erroneously spell it chaperone, app. under the supposition that it requires a fem. termination" [OED]. The notion is of "covering" the socially vulnerable one. The word had been used in Middle English in the literal sense "hooded cloak."

"May I ask what is a chaperon?"
"A married lady; without whom no unmarried one can be seen in public. If the damsel be five and forty, she cannot appear without the matron; and if the matron be fifteen, it will do."

[Catharine Hutton, "The Welsh Mountaineer," London, 1817]
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uppercut (n.)

in pugilism, a close-in strike upward with the fist, 1831, from upper + cut (n.). Perhaps the image is of chopping a tree by making cuts up (as well as down) in the trunk.

It was on a side hill, and I observed a boy, who appeared to be about fifteen years of age, opposite the house felling a large tree; he had cut a few chips from the under side, and was then making the principal incision on the upper. ... I said to the boy, "Well Sir, I see that you make the upper cut." "That is the true cut," said the boy; "for if you will take the axe and try below, you will find that the tree will crowd down upon your chips, and you can't get it down in double the time." [Theodore Sedgwick, "Hints to My Countrymen," 1826]
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