Online Etymology Dictionary

slack (n.2): "coal dust," mid-15c., sleck, probably from Middle Dutch slacke, Middle Low German slecke "slag, small pieces left after coal is screened," perhaps related to slagge "splinter flying off metal when it is struck" (see slag).
slack (adj.): Old English slæc "loose, careless" (in reference to personal conduct), from Proto-Germanic *slakas (cf. Old Saxon slak, Old Norse slakr, Old High German slah "slack," Middle Dutch lac "fault, lack"), from PIE root *(s)leg- "to be slack" (see lax). Sense of "not tight" (in reference to things) is first recorded c.1300. Slack-key (1975) translates Hawaiian ki ho'alu First record of slack-jawed (1901) is in Kipling. Slack water "time when tide is not flowing" is from 1769.
slack (n.1): 1794, "loose part or end" (of a rope, sail, etc.), from slack (adj.); hence figurative senses in take up the slack (1930) and slang cut (someone) some slack (1968). Meaning "quiet period, lull" is from 1851. Slacks "loose trousers" first recorded 1824, originally military.
slack (v.): Old English slacian "to loosen" (something), from the source of slack (adj.). Meaning "relax effort, weaken in trying" is from early 13c.; that of "to diminish in force" is early 14c. Related: Slacked; slacking.
slacken (v.): c.1300, from slack (adj.) + -en (1). Related: Slackened; slackening.
slacker (n.): popularized 1994, but meaning "person who shirks work" dates back to 1898; agent noun from slack (v.).
slackly (adv.): Old English slæclice; see slack (adj.) + -ly (2).
slackness (n.): Old English slæness; see slack (adj.) + -ness.
slag (n.): 1550s, from Middle Low German slagge (German Schlacke) "splinter flying off when metal is struck," related to Old High German slahan "to strike, slay" (see slay).
slag (v.): "denigrate," by 1971, from slag (n.) in sense of "worthless person" (1788). Related: Slagged; slagging.
slain (adj.): early 13c., from Old English (ge)slegen, past participle of slean (see slay). The noun meaning "those who have been slain" is attested from mid-14c.
slake (v.): Old English slacian "slacken an effort," from slæc "lax" (see slack). Sense of "allay" (in reference to thirst, hunger, desire) first recorded early 14c. Related: Slaked; slaking.
slalom (n.): 1921, from Norwegian slalam "skiing race," literally "sloping track," from sla "slope" + lam "track" (related to Norwegian laan "a row of houses;" cf. lane).
slam (n.1): "a severe blow," 1670s, probably from a Scandinavian source (cf. Norwegian slamre, Swedish slemma "to slam, bang") of imitative origin. Slam-bang recorded by 1806. Slam-dunk is from 1976. Slam-dance is attested by 1987 (slam by itself in this sense is recorded from 1983).
slam (n.2): "a winning of all tricks in a card game," 1620s, used especially in whist, of obscure origin. Grand slam in bridge first recorded 1892; earlier in related card games from 1814; figurative sense of "complete success" is attested from 1920; in baseball sense from 1935.
slam (v.): "to shut with force," 1726, from slam (n.1). Meaning "say uncomplimentary things about" is from 1916. Related: Slammed; slamming.
slammer (n.): "jail, prison," 1952, perhaps from earlier U.S. slang sense of "door" (by 1943), agent noun from slam (v.).
slander (n.): late 13c., from Anglo-French esclaundre, Old French esclandre "scandalous statement," alteration of escandle "scandal," from Latin scandalum "cause of offense, stumbling block, temptation" (see scandal).
slander (v.): c.1300, from Old French esclandrer, from esclandre (see slander (n.)). Related: Slandered; slandering.
slang (n.): 1756, "special vocabulary of tramps or thieves," later "jargon of a particular profession" (1801), of uncertain origin, perhaps from a Scandinavian source, cf. Norwegian slengenamn "nickname," slengja kjeften "to abuse with words," literally "to sling the jaw," related to Old Norse slyngva "to sling." But OED, while admitting "some approximation in sense," discounts this connection based on "date and early associations." Liberman also denies it, as well as any connection with French langue. Rather, he derives it elaborately from an old word meaning "narrow piece of land." Sense of "very informal language characterized by vividness and novelty" first recorded 1818. A word that ought to have survived is slangwhanger (1807, American English) "noisy or abusive talker or writer."
slant (n.): 1650s, from slant (v.). Derogatory slang sense of "Oriental, slant-eyed person" is recorded from 1943, from earlier slant-eyes (1929).
slant (v.): 1520s, "to strike obliquely" (against something), alteration of slenten "slip sideways" (c.1300), perhaps via a Scandinavian source (cf. Swedish slinta "to slip," Norwegian slenta "to fall on one side"), from Proto-Germanic *slintanan. Sense of "to slope" is first recorded 1690s. Related: Slanted; slanting. The adjective/adverb is attested from late 15c. Slant rhyme attested from 1944.
slap (v.): 1630s, probably of imitative origin, similar to Low German slappe, German Schlappe. Related: Slapped; slapping.
slap (n.): 1640s, from slap (v.). Figurative meaning "insult" is attested from 1736. Slap-happy (1936) originally meant "punch-drunk." Slapshot in ice hockey is recorded from 1942. Slap on the wrist "very mild punishment" dates from 1914.
slapdash: 1670s (adv.); 1792 (adj.); from slap (v.) + dash (v.).
slapstick (n.): "farcical physical comedy, horseplay," 1926, from slapstick (1896) a device consisting of two sticks fastened together so as to slap loudly when a clown or actor hits somebody with it, or to make a sound-effect offstage; from slap and stick (n.).
slash (v.): late 14c., "to cut with a stroke of a blade or whip," perhaps from Middle French esclachier "to break," variant of esclater "to break, splinter" (see slat). In reference to prices, it is attested from 1906. Related: Slashed; slashing. Slash-and-burn method of clearing forest for cultivation is from 1919.
slash (n.): "a cutting stroke with a weapon," 1570s, from slash (v.); sense of "slit in a garment" is from 1610s; that of "open tract in a forest" is first attested 1825, American English. As a punctuation mark in writing or printing, it is recorded from 1961.
slat (n.): late 14c., "a roofing slate," from Old French esclat "split piece, splinter," back-formation from esclater "to break, splinter, burst," probably from Frankish *slaitan "to tear, slit," related to Old High German slizan, Old English slitan (see slit (v.)). Meaning "long, thin, narrow piece of wood or metal" attested from 1764.
slate (n.): mid-14c., from Old French esclate, fem. of esclat "split piece, splinter" (Modern French éclat; see slat), so called because the rock splits easily into thin plates. As a color, first recorded 1882. Sense of "a writing tablet" (made of slate), first recorded late 14c., led to that of "list of candidates," first recorded 1842. Clean slate (1868) originally referred to scores chalked up in a tavern.
slate (v.): "propose, schedule," 1883; sense of "nominate" is attested from 1804. The notion is of writing on a slate (n.) board. Related: Slated; slating.
slater (n.): c.1400, agent noun from slate.
slather (v.): "spread liberally," 1847, of uncertain origin.
Slather on the manure on all the hoed crops, if you have it; if not buy of your improvident neighbor. ["Genesee Farmer," June 1847]
Sometimes said to be from a dialectal noun meaning "large amount" (usually as plural, slathers), but this is first attested 1855; of uncertain origin, it is perhaps from Irish sliotar. Related: Slathered; slathering.
slattern (n.): 1630s, "a rude, ill-bred woman," probably related to Low German Slattje, Dutch slodder, dialectal Swedish slata "slut" (in the older, non-sexual sense). Cf. dialectal English verb slatter "to spill or splash awkwardly, to waste," used of women or girls considered untidy or slovenly.
slatternly (adj.): 1670s, from slattern + -ly (1).
slaughter (n.): c.1300, "killing of a cattle or sheep for food, killing of a person," from Old Norse *slahtr, akin to slatr "a butchering, butcher meat," slatra "to slaughter," and slattr "a mowing;" related to sla "to strike" (see slay), from Proto-Germanic *slukhtis. Meaning "killing of a large number of persons in battle" is attested from mid-14c.
slaughter (v.): 1530s, from slaughter (n.). Related: Slaughtered; slaughtering.
slaughterhouse (n.): late 14c., from slaughter (n.) + house (n.).
Slav: late 14c., Sclave, from Medieval Latin Sclavus (c.800), from Byzantine Greek Sklabos (c.580), from Old Church Slavonic Sloveninu "a Slav," probably related to slovo "word, speech," which suggests the name originally meant member of a speech community (cf. Old Church Slavonic Nemici "Germans," related to nemu "dumb;" and cf. Old English þeode, which meant both "race" and "language").

Identical with the -slav in personal names (e.g. Russian Miroslav, literally "peaceful fame;" Mstislav "vengeful fame;" Jaroslav "famed for fury;" Czech Bohuslav "God's glory;" and cf. Wenceslas). Spelled Slave c.1788-1866, influenced by French and German Slave.
slave (n.): late 13c., "person who is the property of another," from Old French esclave (13c.), from Medieval Latin Sclavus "slave" (cf. Italian schiavo, French esclave, Spanish esclavo), originally "Slav" (see Slav), so called because of the many Slavs sold into slavery by conquering peoples.
This sense development arose in the consequence of the wars waged by Otto the Great and his successors against the Slavs, a great number of whom they took captive and sold into slavery. [Klein]
Old English Wealh "Briton" also began to be used in the sense of "serf, slave" c.850; and Sanskrit dasa-, which can mean "slave," is apparently connected to dasyu- "pre-Aryan inhabitant of India."

More common Old English words for slave were þeow (related to þeowian "to serve") and þræl (see thrall). The Slavic words for "slave" (Russian rab, Serbo-Croatian rob, Old Church Slavonic rabu) are from Old Slavic *orbu, from the PIE root *orbh- (also source of orphan) the ground sense of which seems to be "thing that changes allegiance" (in the case of the slave, from himself to his master). The Slavic word is also the source of robot.

Applied to devices from 1904, especially those which are controlled by others (cf. slave jib in sailing, similarly of locomotives, flash bulbs, amplifiers). slave-driver is attested from 1807.
slave (v.): c.1600, "to enslave," from slave (n.). The meaning "work like a slave" is first recorded 1719. Related: Slaved; slaving.
Slave: Indian tribe of northwestern Canada, 1789, from slave (n.), translating Cree (Algonquian) awahkan "captive, slave."
slaver (v.): early 14c., from Old Norse slafra "to slaver," probably imitative (see slobber). Related: Slavered; slavering.
slavery (n.): 1550s; from slave + -ery.
Slavic (adj.): 1813; see Slav + -ic. Earlier in same sense was Slavonic (1640s), from Slavonia, a region of Croatia.
slavish (adj.): 1560s, from slave + -ish. Sense of "servilely imitative" is from 1753. Related: Slavishly; slavishness.
slavocracy (n.): also slaveocracy, in U.S. history, "the political dominance of slave-owners," 1840, from slave + -cracy.
slaw (n.): "sliced cabbage," 1794, from Dutch sla, short for salade, from French salade (see salad).
slay (v.): Old English slean "to smite," also "to kill with a weapon" (class VI strong verb; past tense sloh, slog, past participle slagen), from Proto-Germanic *slakhanan, from root *slog- "to hit" (cf. Old Norse and Old Frisian sla, Danish slaa, Middle Dutch slaen, Dutch slaan, Old High German slahan, German schlagen, Gothic slahan "to strike").

The Germanic words are from PIE root *slak- "to strike" (cf. Middle Irish past participle slactha "struck," slacc "sword"). Modern German cognate schlagen maintains the original sense of "to strike." Meaning "overwhelm with delight" (mid-14c.) preserves some of the wider range of meanings that the word once had, including also "to strike a spark" (Old English).
sleave (v.): Old English -slifan "to separate," related to sliver (v.).