Online Etymology Dictionary

seaweed (n.): 1570s, from sea + weed (n.).
sebaceous (adj.): 1728, from Latin sebaceus, from sebum "tallow, grease" (see sebum).
Sebastian: masc. proper name, from Latin Sebastianus, from Greek Sebastianos, "man of Sebastia," a city in Pontus that was named for Augustus Caesar, first Roman emperor, from Greek sebastos "venerable," a translation of Latin augustus, the epithet of Caesar.
seborrhea (n.): 1876, coined in Modern Latin as a hybrid, from sebo-, used as comb. form of Latin sebum "tallow, suet, grease" (see sebum) + Greek hroia "flow, flux."
sebum (n.): 1728, from Latin sebum "suet, grease," probably related to sapo "soap" (see soap (n.)).
sec (n.): 1956, conversational shortening of second (n.).
secant (n.): 1590s, from Latin secantem (nominative secans) "a cutting," present participle of secare "to cut" (see section (n.)). First used by Danish mathematician Thomas Fincke in "Geometria Rotundi" (1583).
secateurs (n.): 1881, from French sécateur, ultimately from Latin secare "to cut" (see section (n.)).
secede (v.): 1702, "to leave one's companions," from Latin secedere (see secession). Sense of "to withdraw from a political or religious alliance of union" is recorded from 1755. Related: Seceded; seceding.
secession (n.): 1530s, from Latin secessionem (nominative secessio), noun of action from past participle stem of secedere "secede," from se- "apart" (see secret) + cedere "to go" (see cede). Originally in a Roman historical context, "temporary migration of plebeians from the city to compel patricians to address their grievances;" modern use in reference to religious or political unions dates from 1650s.
secessionist (n.): 1860, first recorded in U.S. context, from secession + -ist (colloquial short form secesh is attested from 1861); the earlier noun had been seceder, but this had religious overtones, especially in reference to Scottish Church history.
seclude (v.): mid-15c., "to shut up, enclose, confine," from Latin secludere "shut off, confine," from se- "apart" (see secret) + -cludere, variant of claudere "to shut" (see close (v.)). Meaning "to remove or guard from public view" is recorded from 1620s. Related: Secluded; secluding.
secluded (adj.): in reference to places, 1798, past participle adjective from seclude (v.).
seclusion (n.): 1620s, from Medieval Latin seclusionem, noun of action from past participle stem of Latin secludere (see seclude).
seclusive (adj.): 1743, from Latin seclus-, stem of secludere (see seclude) + -ive. Related: Seclusively; seclusiveness.
Seconal: 1935, U.S. proprietary name (Eli Lilly & Co.), from Secon(dary) Al(lyl).
second (adj.): "after first," c.1300, from Old French second, from Latin secundus "following, next in order," from root of sequi "follow" (see sequel). Replaced native other in this sense because of the ambiguousness of the earlier word. Second-rate is from 1660s, originally of ships (see rate); second sight is from 1610s; an etymologically perverse term, because it means in reality the sight of events before, not after, they occur. Second fiddle first attested 1809.
second (n.): "one-sixtieth of a minute," late 14c., from Old French seconde, from Medieval Latin secunda, short for secunda pars minuta "second diminished part," the result of the second division of the hour by sixty (the first being the "prime minute," now called the minute), from Latin secunda, fem. of secundus (see second (adj.)).
second (v.): "to further, to support," 1580s, "to support or represent in a duel, fight, etc.," from Middle French seconder, from Latin secundare "to assist, make favorable," from secundus "assisting, favorable, following, second" (see second (adj.)). the noun in this sense is first recorded 1580s. The verb in the parliamentary sense is first recorded 1590s. Related: Seconded; seconding.
second nature (n.): late 14c., from Latin secundum naturam "according to nature" (Augustine, Macrobius, etc.), literally "following nature;" from medieval Aristotelian philosophy, contrasted to phenomena that were super naturam ("above nature," such as God's grace), extra naturam ("outside nature"), supra naturam ("beyond nature," such as miracles), contra naturam "against nature," etc.
second-class (adj.): 1837, from second (adj.) + class (n.). Phrase second-class citizen is recorded from 1942.
second-guess (v.): 1941, back-formation from second-guesser (1937), American English, originally baseball slang for a fan who loudly questions decisions by players, managers, etc.; perhaps from guesser in the baseball slang sense of "umpire."
secondary (adj.): late 14c., from Latin secundarius "pertaining to the second class, inferior," from secundus (see second (adj.)). Of colors, from 1831; of education, from 1809. Secondary succession attested from 1905.
secondhand (adj.): also second-hand, late 15c., from second (adj.) + hand (n.).
secondly (adv.): late 14c., from second (adj.) + -ly (2).
secondment (n.): 1897, from second (v.) + -ment.
seconds (n.): "articles below the first quality," c.1600, from second (adj.); originally attested in this sense in a Shakespeare sonnet. Meaning "second helping of food at a meal" is recorded from 1792.
secrecy (n.): early 15c., secretee, "quality of being secret," from Old French secré, variant of secret (see secret). Form altered late 16c. on model of primacy, etc.
secret (v.): "to keep secret" (described in OED as "obsolete"), 1590s, from secret (n.). Related: Secreted; secreting.
secret (n.): late 14c., from Latin secretus "set apart, withdrawn, hidden," past participle of secernere "to set apart," from se- "without, apart," properly "on one's own" (from PIE *sed-, from root *s(w)e-; see idiom) + cernere "separate" (see crisis).

As an adjective from c.1400. Secret agent first recorded 1715; secret service is from 1737; secret weapon is from 1936.
secretaire (n.): cabinet for private papers, 1771, from French secréraire, from Medieval Latin secretarius (see secretary). Anglicized form secretary is attested in this sense from 1803.
secretarial (adj.): 1801, from secretary (Medieval Latin secretarius) + -al (1).
secretariat (n.): "office of secretary," 1811, from French secrétariat, from Medieval Latin secretarius (see secretary).
secretary (n.): late 14c., "person entrusted with secrets," from Medieval Latin secretarius "clerk, notary, confidential officer, confidant," from Latin secretum "a secret" (see secret). Meaning "person who keeps records, write letters, etc.," originally for a king, first recorded c.1400. As title of ministers presiding over executive departments of state, it is from 1590s. The word also is used in both French and English to mean "a private desk," sometimes in French form secretaire (1818).
secrete (v.): 1707, back-formation from secretion. Related: Secreted; secretes; secreting.
secretion (n.): 1640s, from French sécrétion, from Latin secretionem (nominative secretio) "separation," noun of action from past participle stem of secernere "to separate, set apart" (see secret).
secretive (adj.): "inclined to secrecy," 1815 (implied in secretiveness); see secret + -ive. Related: Secretively.
secretory: 1690s; see secrete + -ory.
sect (n.): c.1300, "distinctive system of beliefs or observances; party or school within a religion," from Old French secte, from Late Latin secta "religious group, sect," from Latin secta "manner, mode, following, school of thought," literally "a way, road," from fem. of sectus, variant past participle of sequi "follow," from PIE *sekw- "to follow" (see sequel). Confused in this sense with Latin secta, fem. past participle of secare "to cut" (see section). Meaning "separately organized religious body" is recorded from 1570s.
sectarian (adj.): 1640s, originally applied by Presbyterians to Independents, from Medieval Latin sectarius, from secta (see sect).
sectarianism (n.): 1670s, "disposition to petty sects in opposition to things established" [Johnson]; see sectarian + -ism.
sectary (n.): 1550s, from French sectaire or directly from Medieval Latin sectarius, from secta (see sect).
section (n.): 1550s, from Middle French section, from Latin sectionem (nominative sectio) "a cutting, cutting off, division," noun of action from past participle stem of secare "to cut," from PIE root *sek- "cut" (cf. Old Church Slavonic seko, sesti "to cut," secivo "ax, hatchet;" Lithuanian isekti "to engrave, carve;" Albanian sate "mattock;" Old Saxon segasna, Old English sigðe "scythe;" Old English secg "sword," seax "knife, short sword;" Old Irish doescim "I cut;" Latin saxum "rock, stone").
sectional (adj.): 1806; see section (n.) + -al. Noun meaning "piece of furniture composed of sections which can be used separately" is attested by 1961, from sectional seat, sectional sofa, etc. (1949).
sectionalism (n.): 1858, from sectional + -ism.
sector (n.): 1570, "section of a circle between two radii," from Late Latin sector "section of a circle," from Latin sector "a cutter," from sectus, past participle of secare "to cut" (see section (n.)). Translated Greek tomeus in Latin editions of Archimedes. Meaning "area, division" appeared 1920, generalized from military sense (1916) of "part of a front," based on a circle centered on a headquarters.
secular (adj.): late 13c., "living in the world, not belonging to a religious order," also "belonging to the state," from Old French seculer, from Late Latin saecularis "worldly, secular," from Latin saecularis "of an age, occurring once in an age," from saeculum "age, span of time, generation," probably originally cognate with words for "seed," from PIE root *se(i)- "to sow" (cf. Gothic mana-seþs "mankind, world," literally "seed of men").

Used in ecclesiastical writing like Greek aion "of this world" (see cosmos). It is source of French siècle. Ancient Roman ludi saeculares was a three-day, day-and-night celebration coming once in an "age" (120 years).
secularism (n.): "doctrine that morality should be based on the well-being of man in the present life, without regard to religious belief or a hereafter," 1846, from secular + -ism.
secularist (n.): 1851, "adherent of secularism;" see secularism + -ist.
secularization (n.): 1706; see secularize + -ation.