Etymology
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crusade (n.)

"military expedition under the banner of the cross," 1706, a respelling or replacement of croisade (1570s), from French croisade (16c.), Spanish cruzada, both from Medieval Latin cruciata, past participle of cruciare "to mark with a cross," from Latin crux (genitive crucis) "cross" (see crux).

The modern English form is comparatively late, and even the earlier croisade is post-Middle English (French croisade replaced earlier croisée). Middle English nouns were croiserie (c. 1300), creiserie.

Especially in reference to the medieval expeditions undertaken by European Christians for recovery of the Holy Land from Muslims. Generally they are counted as seven between 1095 and 1271, but some smaller efforts (e.g. the "Children's Crusade") are omitted and the word sometimes is extended to other religiously motivated expeditions (e.g. against the Albigenses or the Prussians). Figurative sense of "vigorous campaign for a moral cause or against a public evil" is from 1786.

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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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crusade (v.)

1732, "to engage in a crusade," from crusade (n.). The usual way to express this in Middle English seems to have been take the cross (c. 1300). Related: Crusaded; crusading.

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crusader (n.)

person engaged in a crusade, 1743, from crusade (n.) + -er (1). Earlier was croisader, from French croisadeur. In Middle English a crusader might be called a pilgrim.

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Sextus 

masc. proper name, from Latin, properly "the sixth," originally denoting a sixth child, from sextus "sixth," from sex "six" (see six; compare Octavian).

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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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