Etymology
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slay (v.)

Middle English slēn, from Old English slean "to smite, strike, beat," also "to kill with a weapon, slaughter" (class VI strong verb; past tense sloh, slog, past participle slagen), from Proto-Germanic *slahanan "to hit" (source also of 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 said to be from PIE root *slak- "to strike" (source also of Middle Irish past participle slactha "struck," slacc "sword"), but, given certain phonetic difficulties and that the only cognates are Celtic, Boutkan says the evidences "point to a North European substratum word."

The verb slēn displays many nondialectal stem variants because of phonological changes and analogical influences both within its own paradigm and from other strong verbs. [Middle English Compendium]

Modern German cognate schlagen maintains the original sense of "to strike." Meaning "overwhelm with delight" (mid-14c.) preserves one of the wide range of meanings the word once had, including, in Old English, "stamp (coins); forge (weapons); throw, cast; pitch (a tent), to sting (of a snake); to dash, rush, come quickly; play (the harp); gain by conquest."

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slay (n.)
"instrument on a weaver's loom to beat up the weft," Old English slæ, slea, slahae, from root meaning "strike" (see slay (v.)), so called from "striking" the web together. Hence the surname Slaymaker "maker of slays."
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slayer (n.)
late 14c., agent noun from slay (v.). The Old English agent noun was slaga "slayer, killer."
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slug (n.3)
"a hard blow," 1830, dialectal, of uncertain origin; perhaps related to slaughter or perhaps a secondary form of slay.
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slain (adj.)

early 13c., from Old English (ge)slegen, past participle of slean "to smite, strike, beat" (see slay (v.)). The noun meaning "those who have been slain" is attested from mid-14c.

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slag (n.)
"refuse from smelting," 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 (v.)).
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sledge (n.1)
"heavy hammer," Old English slecg "hammer, mallet," from Proto-Germanic *slagjo- (source also of Old Norse sleggja, Middle Swedish sleggia "sledgehammer"), related to slege "beating, blow, stroke" and slean "to strike" (see slay (v.)). Sledgehammer is pleonastic.
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schlock (n.)
"trash," 1915, from American Yiddish shlak, from German Schlacke "dregs, scum, dross" (see slag (n.)). Alternative etymology [OED] is from Yiddish shlogn "to strike" (cognate with German schlagen; see slay). Derived form schlockmeister "purveyor of cheap merchandise" is from 1965. Adjectival form schlocky is attested from 1968; schlock was used as an adjective from 1916.
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slaughter (n.)

c. 1300, "the killing of a person, murder; the killing of large numbers of persons in battle;" mid-14c., "the killing of a cattle or sheep for food;" from a Scandinavian source akin to Old Norse slatr "a butchering, butcher meat," slatra "to slaughter," slattr "a mowing," related to Old Norse sla "to strike," from Proto-Germanic *slagan- (see slay (v.)).

The form was perhaps influenced by Middle English slaught "killing, manslaughter, carnage; butchery of animals," the native cognate, from Old English sliht, sleht, slieht "stroke, slaughter, murder, death; animals for slaughter;" as in sliehtswyn "pig for killing."

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sly (adj.)
c. 1200, "skillful, clever, dexterous," from Old Norse sloegr "cunning, crafty, sly," from Proto-Germanic *slogis (source also of Low German slu "cunning, sly," German schlau), probably from base *slak- "to strike, hit" (see slay (v.)), with an original notion of "able to hit." Compare German verschlagen "cunning, crafty, sly," schlagfertig "quick-witted," literally "strike-ready," from schlagen "to strike." A non-pejorative use of the word lingered in northern English dialect until 20c. On the sly "in secret" is recorded from 1812. Sly-boots "a seeming Silly, but subtil Fellow" is in the 1700 "Dictionary of the Canting Crew."
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