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."
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.
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."
"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. A common Germanic word; compare German bleuen, Gothic bliggwan "to strike."
Influenced in English by blow (v.1). 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]