Online Etymology Dictionary

seventh: c. 1300, from seven + -th (1); earlier sevende, seveth, from Old English seofunda (Anglian, Northumbrian), seofoþa (West Saxon), from Proto-Germanic *sebundon (source also of Old Norse sjaundi, Old Saxon sivondo, Old High German sibunto, German siebente, siebte). The music note sense is from 1590s.
seventies (n.): 1859 as the years of someone's life between 70 and 79; from 1837 as the eighth decade of years in a given century. See seventy.
seventieth (adj.): late 13c., from seventy + -th (1).
seventy: Old English (hund)seofontig, from seofon (see seven) + -tig (see -ty (1)). Similar formation in Old Frisian soventich, Middle Dutch seventich, Old Norse sjau tiger.
sever (v.): c. 1300, from Anglo-French severer, Old French sevrer "to separate" (12c., later in French restricted to "to wean," i.e. "to separare from the mother"), from Vulgar Latin *seperare, from Latin separare "to separate" (see separate (v.)).
several (adj.): early 15c., "existing apart," from Anglo-French several, from Middle French seperalis "separate," from Medieval Latin separalis, from Latin separ "separate, different," back-formation from separare "to separate" (see separate (v.)). Meaning "various, diverse, different" is attested from c. 1500; that of "more than one" is from 1530s, originally in legal use.
Here we are all, by day; by night we're hurled
By dreams, each one into a several world
[Herrick, 1648]
Related: Severalty. Jocular ordinal form severalth attested from 1902 in American English dialect (see -th (2)).
severally (adv.): "separately," late 14c., from several + -ly (2).
severance (n.): early 15c., from Anglo-French, from Old French sevrance "separation, parting," from sevrer (see sever). Meaning "discharge from employment contract" is attested from 1941. Severance pay attested by 1942.
severe (adj.): 1540s, from Middle French severe (12c., Modern French sévère) or directly from Latin severus "serious, grave, strict, austere" (see severity). From 1660s with reference to styles or tastes; from 1725 of diseases.
severely (adv.): 1540s, from severe + -ly (2). Colloquial sense of "excessively" attested by 1854.
severity (n.): late 15c., "austerity or strictness of life," from Middle French severite, from Latin severitas "seriousness, strictness, sternness," from severus "stern, strict, serious," of uncertain origin. Possibly from PIE root *segh- "to have, hold" (see scheme (n.)), or possibly from *se vero "without kindness," from se "without" (see secret) + *vero "kindness," neuter ablative of verus "true" (see very). Meaning "strictness in dealing with others" is recorded from 1520s.
Seville: inland port city in Spain, Spanish Sevilla, ultimately from Phoenician, from sefela "plain, valley."
sew (v.): Old English siwian "to stitch, sew, mend, patch, knit together," earlier siowian, from Proto-Germanic *siwjanan (source also of Old Norse syja, Swedish sy, Danish sye, Old Frisian sia, Old High German siuwan, Gothic siujan "to sew"), from PIE root *syu- "to bind, sew" (source also of Sanskrit sivyati "sews," sutram "thread, string;" Greek hymen "thin skin, membrane," hymnos "song;" Latin suere "to sew, sew together;" Old Church Slavonic šijo "to sew," šivu "seam;" Lettish siuviu, siuti "to sew," siuvikis "tailor;" Russian švec "tailor"). Related: Sewed; sewing. To sew (something) up "bring it to a conclusion" is a figurative use attested by 1904.
sewage (n.): 1818, from sew (v.) "to drain, draw off water" (late 15c., from sewer (n.1)) + -age.
sewer (n.1): c. 1400, "conduit," from Anglo-French sewere, Old North French sewiere "sluice from a pond" (13c.), literally "something that makes water flow," from shortened form of Gallo-Roman *exaquaria (source of Middle French esseveur), from Latin ex- "out" (see ex-) + aquaria, fem. of aquarius "pertaining to water," from aqua "water" (see aqua-).

Specifically of underground channels for wastewater from c. 1600; figurative use of this is from 1640s.
sewer (n.2): "one who sews," late 14c., agent noun from sew (v.).
sewerage (n.): 1832, from sewer (n.1) + -age.
sewing (n.): late 13c., "action of sewing;" c. 1400, "sewn work," verbal noun from sew (v.). Sewing machine is attested from 1847.
sex (v.): 1884, "to determine the sex of," from sex (n.); to sex (something) up "increase the sex appeal of" is recorded from 1942. Related: Sexed; sexing.
sex (n.): late 14c., "males or females collectively," from Latin sexus "a sex, state of being either male or female, gender," of uncertain origin. "Commonly taken with seco as division or 'half' of the race" [Tucker], which would connect it to secare "to divide or cut" (see section (n.)). Meaning "quality of being male or female" first recorded 1520s. Meaning "sexual intercourse" first attested 1929 (in writings of D.H. Lawrence); meaning "genitalia" is attested from 1938. Sex appeal attested by 1904.
For the raw sex appeal of the burlesque "shows" there is no defense, either. These "shows" should be under official supervision, at the least, and boys beneath the age of eighteen forbidden, perhaps, to attend their performance, just as we forbid the sale of liquors to minors. [Walter Prichard Eaton, "At the New Theatre and Others: The American Stage, Its Problems and Performances," Boston, 1910]
Sex drive is from 1918; sex object is 1901; sex symbol is 1871 in anthropology; the first person to whom the term was applied seems to have been Marilyn Monroe (1959). Sex therapist is from 1974.
sexagenarian (n.): 1738, "person sixty years old," from Latin sexagenarius "containing sixty," from sexagenarius, from sexageni "sixty each, sixty at a time," from sexaginta "sixty," from comb. form of sex (see six) + -genaria "ten times," from -ginta "tens," from PIE *dkm-ta-, from *dekm- "ten" (see ten). As an adjective from 1836.
sexagesimal (adj.): "pertaining to 60," 1680s, from Medieval Latin sexagesimalis, from Latin sexagesimus "the sixtieth," from sexaginta "sixty."
sexiness (n.): 1922, from sexy + -ness.
sexism (n.): 1968; see sexist + -ism.
sexist (adj.): 1965, from sex (n.) on model of racist, coined by Pauline M. Leet, director of special programs at Franklin & Marshall College, Lancaster, Pennsylvania, U.S., in a speech which was circulated in mimeograph among feminists. Popularized by use in print in Caroline Bird's introduction to "Born Female" (1968).
sexless (adj.): 1590s, from sex (n.) + -less. Related: Sexlessly; sexlessness.
sexology (n.): 1902, from sex (n.) + -ology. Related: Sexologist.
sexpert (n.): "sex therapist," 1924, from jocular merger of sex (n.) + expert.
sexploitation (n.): 1942, from sex (n.) + exploitation. Other similar coinages include sexpert (1924); sexcapade (1953); sexational (1927); and sexophone in "Brave New World."
sexpot (n.): "erotically willing and desirable female," 1929, from sex (n.) + pot (n.1), perhaps suggested by fleshpot.
sext (v.): by 2005, from contraction of sex (n.) + text (v.). Related: Sexted; sexting.
sext (n.): early 15c., "third of the lesser canonical hours," from Latin sexta (hora), fem. of sextus, ordinal of sex (see six).
sextant (n.): instrument for determining latitude, 1620s, from Modern Latin sextans, said to have been coined c. 1600 by Danish astronomer Tycho Brahe, from Latin sextans "a sixth," from sex "six" (see six). So called because the sextans has a graduated arc equal to a sixth part of a circle.
sextet (n.): 1841, altered (by influence of German Sextett) from sestet.
sextile: 1550s (adj.), "at a distance of 60 degrees;" 1590s (n.); from Latin sextilis (adj.) "the sixth," from sextus "sixth" (ordinal number; see Sextus).
sextillion (n.): 1680s, from Latin sext-, comb. form of sex "six" (see six) + (m)illion. Compare billion. Related: Sextillionth.
sexton (n.): c. 1300, sekesteyn, "person in charge of the sacred objects of a church," from Old French segrestien, from Medieval Latin sacristanus (see sacristan). Sense of "custodian of a church" first recorded 1580s. Fem. forms sextress, sextrice are recorded 15c., but the usual form is sextoness (early 15c.).
sextuple (adj.): "sixfold," 1620s, from Latin sextus "sixth" (from sex "six;" see six) + -plus "more" (see plus).
sextuplet (n.): 1852, from adjective sextuple (1620s); patterned on triplet, etc.
Sextus: masc. proper name, from Latin, properly "the sixth," originally denoting a sixth child, from sextus "sixth," from sex "six" (see six; also see Octavian).
sexual (adj.): 1650s, "of or pertaining to the fact of being male or female," from Late Latin sexualis "relating to sex," from Latin sexus (see sex (n.)). Meaning "pertaining to copulation or generation" is from 1766; sexual intercourse attested by 1771; sexual orientation by 1967; sexual harassment by 1975. Sexual revolution attested by 1962. Sexual politics is from 1970. Related: Sexually.
sexuality (n.): 1789, "action or fact of being sexual;" see sexual + -ity. Meaning "capability of sexual feelings" is from 1879. Meaning "sexual identity" is by 1980.
sexualization (n.): 1872, noun of action from sexualize.
sexualize (v.): 1839, from sexual + -ize. Related: Sexualized; sexualizing.
sexy (adj.): 1905, from sex (n.) + -y (2). Originally "engrossed in sex;" sense of "sexually attractive" is 1923, first in reference to Valentino. An earlier word in this sense was sexful (1898). Related: Sexier; sexiest.
Seychelles: renamed 1756 in honor of French finance minister Jean Moreau de Séchelles; spelling altered 1794 by the English when they took the islands from France. Related: Seychellois.
sforzando (adj.): "with sudden energy or impulse," 1801, from Italian sforzando, gerundive of sforza "to force" (see effort).
sfumato (adj.): 1847, from Italian sfumato, literally "smoked," from Latin fumus "smoke" (see fume (n.)).
sh (interj.): exclamation used to urge or request silence, 1847. The gesture of putting a finger to the lips to express silence is attested from Roman times. As a transitive verb from 1887; intransitive from 1925.
sh-: sound represented in Old English by -sc- (fisc "fish"), which originally was pronounced "-sk-" but which by late Old English had softened to "-sh-." Modern English words with -sc- mostly are imports (generally Scandinavian).

The "sh" sound did not exist in Old French, therefore French scribes after the Norman conquest often represented it with -ssh- in medial and final positions, and sch- in initial positions (schape, schamful, schaft for shape, shameful, shaft). But the spelling -sh- has been standard since Caxton, probably as a worn-down form of Middle English -sch-.

In some East Anglian texts from 14c.-15c., x- is used (xal, xulde for shall, should), which would have given the language a very different look had it prevailed, but the London-based sh- ended up as the standard form. The same Germanic sound has become, by natural evolution, modern German and Dutch sch-, Scandinavian sk-.