Middle English slēn, "strike, beat, strike so as to kill, commit murder," 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."
It is attested by late 12c. as "destroy, put an end to." The 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."
"instrument on a weaver's loom to beat up the weft," Middle English sleie, from Old English slæ, slea, slahae "a weaver's reed," from root meaning "strike" (see slay (v.)), so called from "striking" the web to compress it. Hence also the surname Slaymaker "maker of slays."
"that has been killed," 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.
"refuse matter 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.)).
"heavy hammer," formerly the largest hammer used in forges or by smiths, typically requiring two hands to wield, Middle English slegge, from Old English slecg "hammer, mallet," from Proto-Germanic *slagjo- (source also of Old Norse sleggja, Middle Swedish sleggia "sledgehammer;" German Schlage "tool for striking"), which is related to slege "beating, blow, stroke" and slean "to strike" (see slay (v.)). Sledgehammer is pleonastic.
c. 1300, "the killing of a person, murder; the killing of large numbers of persons in battle;" mid-14c., "the killing of cattle or sheep or other animals 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 obsolete slaught "killing, manslaughter, carnage; butchery of animals," the native cognate, which is from Old English sliht, sleht, slieht "stroke, slaughter, murder, death; animals for slaughter;" as in sliehtswyn "pig for killing." The Elizabethans had an adjective slaughterous.
"a hard hit (with a fist)," mid-15c., blaw, blowe, from northern and East Midlands dialects, perhaps from Middle Dutch blouwen "to beat," or an unrecorded Old English cognate. The ordinary Old English word for "to strike" was slean (see slay (v.)). A common Germanic word; compare German bleuen, Gothic bliggwan "to strike."
Influenced in English by blow (v.1). The figurative sense of "a sudden shock or calamity" is from 1670s. To come to blows "engage in combat" is from 1650s (fall to blows is from 1590s). In reference to descriptions or accounts, blow-by-blow is recorded from 1921, American English, originally of detailed accounts in prize-fight broadcasts.
LIKE a hungry kitten loves its saucer of warm milk, so do radio fans joyfully listen to the blow-by-blow broadcast description of a boxing bout. [The Wireless Age, December 1922]
late 12c., sleigh, "skillful, clever, dexterous, wise, prudent," from Old Norse sloegr "cunning, crafty, sly," from Proto-Germanic *slogis (source also of Low German slu "cunning, sly," German schlau), probably from *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," but the exact relation is uncertain.
The meaning "insidious, crafty, meanly artful" is from c. 1200. That of "playfully artful, knowing" is from 1764. In Middle English sly words could mean "wise words" or "deceptive language." 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."