"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.
Entries linking to slain
slay v.
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."
born adj.
Old English boren, alternative past participle of beran (see bear (v.)). The -en of the Middle English past participles tended to drop the -e- in some verbs, especially after vowels, -r-, and -l- , hence also slain, etc., Middle English stoln. "In modern use the connexion with bear is no longer felt; the phrase to be born has become virtually an intr. verb" [OED].
It is attested from early 14c. as "possessing from birth the character or quality described" (born poet, born loser, etc.). It is from 1710 as "innate, inherited;" the colloquial expression in (one's) born days "in (one's) lifetime" is by 1742. The distinction of born from borne (q.v.) is 17c.