Online Etymology Dictionary

semicircular (adj.): early 15c., from Latin semicirculus (see semicircle) + -ar.
semicolon (n.): punctuation-mark, 1640s, a hybrid coined from Latin-derived semi- + Greek-based colon (n.1). The mark itself was (and is) in Greek the point of interrogation.
semiconductor (n.): 1838, "material whose electrical conductivity is between that of a conductor and that of an insulator," from semi- + conductor. Modern very specific sense is recorded from 1931.
semiconscious (adj.): also semi-conscious, 1838, from semi- + conscious. Related: Semiconsciously; semiconsciousness.
semifinal (adj.): also semi-final, 1867, from semi- + final. As a noun from 1868.
seminal (adj.): late 14c., "of seed or semen," from Old French seminal (14c.) and directly from Latin seminalis, from semen (genitive seminis) "seed" (see semen). Figurative sense of "full of possibilities" is attested from 1630s. Related: Seminally; seminality.
seminar (n.): 1887, "special group-study class for advanced students," from German Seminar "group of students working with a professor," from Latin seminarium "breeding ground, plant nursery" (see seminary). Sense of "meeting for discussion of a subject" first recorded 1944.
seminarian (n.): "seminary student," 1580s, from seminary + -ian.
seminary (n.): mid-15c., "plot where plants are raised from seeds," from Latin seminarium "plant nursery, seed plot," figuratively, "breeding ground," from seminarius "of seed," from semen (genitive seminis) "seed" (see semen). Meaning "school for training priests" first recorded 1580s; commonly used for any school (especially academies for young ladies) from 1580s to 1930s.
semination (n.): 1530s, "action of sowing, from Latin seminationem (nominative seminato) "a sowing, propagation," noun of action from past participle stem of seminare "to plant, propagate," from semen (genitive seminis) "seed" (see semen).
Seminole (n.): 1763, from Creek (Muskogean) simano:li, earlier simalo:ni "wild, untamed, runaway," from American Spanish cimarron (see maroon (v.)). They fought wars against U.S. troops 1817-18 and 1835-42, after which they largely were removed to Indian Territory (Oklahoma).
semiology (n.): 1690s, "sign language," from Greek semeion "a sign, mark, token," from sema (compare semiotic) + -ology. As "branch of medical science concerned with symptoms," 1839; as "logical theory of signs" from 1923. Related: Semiological.
semiotic (adj.): 1620s, "of symptoms, relating to signs of diseases," from Greek semeiotikos "significant," also "observant of signs," adjective form of semeiosis "indication," from semeioun "to signal, to interpret a sign," from semeion "a sign, mark, token," from sema "sign" (see semantic). Its use in psychology dates to 1923. Related: Semiotical (1580s).
semiotics (n.): study of signs and symbols with special regard to function and origin, 1880, from semiotic; also see -ics. Medical sense is from 1660s.
semiprecious (adj.): also semi-precious, 1818, from semi- + precious (adj.).
semiquaver (n.): "sixteenth-note," 1570s, from semi- + quaver (n.).
semisweet (adj.): also semi-sweet, 1943, from semi- + sweet.
Semite (n.): 1847, "a Jew, Arab, Assyrian, or Aramaean" (an apparently isolated use from 1797 refers to the Semitic language group), back-formation from Semitic or else from French Sémite (1845), from Modern Latin Semita, from Late Latin Sem "Shem," one of the three sons of Noah (Gen. x:21-30), regarded as the ancestor of the Semites (in old Bible-based anthropology), from Hebrew Shem. In modern sense said to have been first used by German historian August Schlözer in 1781.
Semitic (adj.): 1797, denoting the language group that includes Hebrew, Arabic, Aramaic, Assyrian, etc.; 1826 as "of or pertaining to Semites," from Medieval Latin Semiticus (source of Spanish semitico, French semitique, German semitisch), from Semita (see Semite). As a noun, as the name of a linguistic family, from 1813. In non-linguistic use, perhaps directly from German semitisch. In recent use often with the specific sense "Jewish," but not historically so limited.
Semitism (n.): 1848, "characteristic attributes of Semitic languages;" 1851, "characteristic attributes of Semitic people," from Semite + -ism. From 1870 as "Jewish influence in a society."
semitone (n.): c. 1600, from semi- + tone (n.) in the musical sense.
semolina (n.): meal from hard kernels of wheat, 1797, alteration of Italian semolino "grits; paste for soups," diminutive of semola "bran," from Latin simila "the finest flour," probably from the same Semitic source as Greek semidalis "the finest flour" (compare Assyrian samidu, Syrian semida "fine meal").
semper idem: Latin, literally "always the same;" see semper- + identical.
semper-: word-forming element meaning "always, ever," from Latin semper "always, ever, at all times, continuously" (literally "once for all"), from PIE *sem- (1) "one, as one, together" (source also of Latin semel "once," similis "like;" see same).
sempiternal (adj.): c. 1400, from Old French sempiternel "eternal, everlasting" (13c.) or directly from Late Latin sempiternalis, from Latin sempiternus "everlasting," from semper "always, ever" (see semper-) + aeternus "eternal" (see eternal). Related: Sempiternally.
senate (n.): c. 1200, "legal and administrative body of ancient Rome," from Old French senat or Latin senatus "highest council of the state in ancient Rome," literally "council of elders," from senex (genitive senis) "old man, old" (see senile). Attested from late 14c. in reference to governing bodies of free cities in Europe; of national governing bodies from 1550s; specific sense of upper house of U.S. legislature is recorded from 1775.
senator (n.): c. 1200, "member of an (ancient) senate," from Old French senator (Modern French sénateur), from Latin senator "member of the senate," from senex "old; old man" (see senate). An Old English word for one was folcwita. As "member of a (modern) governing body" from late 14c.; specifically in U.S. use from 1788. Fem. form senatress attested from 1731. The Senators was the name of the professional baseball team in Washington, D.C., from 1891 to 1971.
senatorial (adj.): 1740, from French sénatorial or from Latin senatorius "pertaining to a senator" or formed in English from senator + -al (1). Earlier adjectives were senatory (1520s), senatorian (1610). Related: Senatorially.
send (v.): Old English sendan "send, send forth; throw, impel," from Proto-Germanic *sandijan (source also of Old Saxon sendian, Old Norse and Old Frisian senda, Middle Low German and Middle Dutch senden, Dutch zenden, German senden, Gothic sandjan), causative form of base *sinþan, denoting "go, journey" (source of Old English sið "way, journey," Old Norse sinn, Gothic sinþs "going, walk, time"), from PIE root *sent- "to head for, go" (source also of Lithuanian siusti "send;" see sense (n.)).

Also used in Old English of divine ordinance (as in godsend, from Old English sand "messenger, message," from Proto-Germanic *sandaz "that which is sent"). Slang sense of "to transport with emotion, delight" is recorded from 1932, in American English jazz slang.
send-off (n.): "a farewell" (especially a funeral), 1872, from verbal phrase (attested by 1660s), from send (v.) + off (adv.).
send-up (n.): "a spoof," British slang, 1958, from verbal phrase send up "to mock, make fun of" (1931), from send (v.) + up (adv.), perhaps a transferred sense of the public school term for "to send a boy to the headmaster" (usually for punishment), which is attested from 1821.
sended: alternative past tense and past participle of send. Attested since late 14c.
sender (n.): c. 1200, agent noun from send (v.). In 1930s slang, a popular musician or song. Sendee is recorded from 1806.
Seneca: 1610s, from Dutch Sennecas, collective name for the Iroquois tribes of what became upper New York, of uncertain origin, perhaps from a Mahican name for the Oneida or their village. Earlier sinnekens, senakees; form probably influenced by the name of the ancient Roman philosopher.
Senegal: African nation, named for the river through it, perhaps from a local word meaning "navigable."
senescence (n.): 1690s, from senescent + -ence. Related: Scenescency (1660s).
senescent (adj.): 1650s, from Latin senescentem (nominative scenescens), present participle of senescere "to grow old," from senex "old" (see senile).
seneschal (n.): late 14c., "steward, majordomo, officer in a royal household in charge of ceremonies and feasts," from Old French seneschal, title of a high administrative court officer, from Frankish Latin siniscalcus, from Proto-Germanic *sini-skalk "senior servant;" first element cognate with Latin senex "old" (see senile); second element from Proto-Germanic *skalkoz "servant" (source also of Gothic skalks, Old High German scalc, Old English scealc "servant;" see second element of marshal).
senicide (n.): "killing of the old men," 1889, from stem of Latin senex ""old man" (see senile) + -cide.
senile (adj.): 1660s, "suited to old age," from French sénile (16c.), from Latin senilis "of old age," from senex (genitive senis) "old, old man," from PIE root *sen- "old" (source also of Sanskrit sanah "old;" Avestan hana- "old;" Old Persian hanata- "old age, lapse of time;" Armenian hin "old;" Greek enos "old, of last year;" Lithuanian senas "old," senis "an old man;" Gothic sineigs "old" (used only of persons), sinistra "elder, senior;" Old Norse sina "dry standing grass from the previous year;" Old Irish sen, Old Welsh hen "old"). Meaning "weak or infirm from age" is first attested 1848.
senility (n.): 1753, from senile + -ity.
senior (adj.): late 13c., from Latin senior "older," comparative of senex (genitive senis) "old," from PIE root *sen- "old" (see senile). Original use in English was as an addition to a personal name indicating "the father" when father and son had the same name; meaning "higher in rank, longer in service" first recorded 1510s.

The Latin word yielded titles of respect in many languages, such as French sire, Spanish señor, Portuguese senhor, Italian signor. Senior citizen first recorded 1938, American English.
senior (n.): mid-14c., "person of authority;" late 14c., "person older than another," from senior (adj.). Sense of "fourth-year student" is from 1741, from earlier general sense of "advanced student" (1610s).
seniority (n.): "priority on office or service," mid-15c., from Medieval Latin senioritas, from Latin senior (see senior (adj.)). Meaning "state or quality of being senior" is from 1530s.
senna (n.): tropical shrub, 1540s, from Modern Latin senna, from Arabic sana. Earlier was sene (c. 1400), from French.
sennight (n.): "period of seven days, a week" (archaic), c. 1200, contracted from Old English seofon nihta; see seven + night. Also compare fortnight.
senor: 1620s, from Spanish señor "a gentleman; sir," from Latin seniorem (source also of Portuguese senhor; see senior (adj.)).
senora: 1570s, from Spanish señora "a lady; madam," fem. of señor (see senor). The Portuguese equivalent is senhora.
senorita (n.): "a young Spanish lady," 1823, from Spanish señorita, Spanish title corresponding to English "Miss," diminutive of señora (see senora). The Portuguese equivalent is senhorita.
senryu: form of Japanese poetry, 1901, from name of Karai Senryu (1718-90), Japanese poet.