Etymology
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sixth (adj., n.)

"next in order after the fifth; an ordinal numeral; being one of six equal parts into which a whole is regarded as divided;" 1520s, replacing Middle English sixte (c. 1200), from Old English syxte, from siex (see six). Compare Old Frisian sexta, Middle Dutch seste, Old High German sehsto, German sechste, Gothic saihsta. With ending conformed to -th (1). Related: Sixthly. The noun meaning "a sixth part" is from 1550s. As a music tone, from 1590s. Sixth sense "supernatural perception of objects" is attested from 1712; earlier it meant "titillation, the sense that apprehends sexual pleasure" (1690s, from Scaliger).

Then said Peter, That is false; for there is a sixth Sense, that of Prescience : for the other five Senses are capable only of Knowledg ; but the Sixth of Foreknowledg ; which Sense the Prophets had. [William Whitson, "Primitive Christianity Reviv'd," vol. v, London, 1712]
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zeta (n.)
sixth letter of the Greek alphabet; see zed.
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sextant (n.)

instrument for determining latitude in navigation and surveying, 1620s, from Modern Latin sextans, which is said to have been first used in this sense c. 1600 by Danish astronomer Tycho Brahe, from Latin sextans "a sixth, a sixth part," from sex "six" (see six). So called because the sextans has a graduated arc equal to a sixth part of a circle. In ancient Rome, sextans also was the name of a coin of the republic worth one-sixth of an as. Related: Sextantal.

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siesta (n.)
"mid-day nap," 1650s, from Spanish siesta, from Latin sexta (hora) "sixth (hour)," the noon of the Roman day (coming six hours after sunrise), from sexta, fem. of sextus "sixth" (see Sextus).
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bissextile 

1580s (n.); 1590s (adj.), in reference to Roman leap year, from Late Latin (annus) bisextilis "leap year," more literally "the twice sixth-day, (a year) containing a second sixth (day)." To keep the Julian calendar consistent with the sun, the sixth day (by inclusive reckoning) before the Calends of March was doubled every four years. The date corresponds to our February 24th. From Latin bissextus/bisextis (dies), from bis "twice" (see bis-) + sextus "sixth (day before the First of March)," from sex "six" (see six).

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la (1)
musical note (sixth note of the diatonic scale), early 14c., see gamut. It represents the initial syllable of Latin labii "of the lips." In French and Italian it became the name of the musical note A, which is the sixth of the natural scale (C major).
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sext (n.)

early 15c., "third of the lesser canonical hours" in churches and religious houses, from Latin sexta (hora), fem. of sextus, ordinal of sex (see six). The office of the sixth hour, originally and properly said at midday. Also "the interval of a sixth in music," etc.

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sextile (adj.)

late 14c., in astrology, of two planets, "at an angular distance of 60 degrees;" 1590s (n.); from Latin sextilis (adj.) "the sixth" (in classical Latin used only in the calendar, with mensis, as the old name of August); from sextus "sixth," from sex "six" (see six).

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fifties (n.)
1826 as the years of someone's life between 50 and 59; 1853 as the sixth decade of years in a given century. See fifty.
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June 
sixth month, c. 1300, Iun, June, Juin, from Latin Iunius (mensis), probably a contraction of Iunonius, "sacred to Juno" (see Juno). Replaced Old English liðe se ærra "earlier mildness." Spelling variant Iune lingered until 17c.
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