Lethal
late 16th century (in the sense ‘causing spiritual death’): from Latin lethalis, from lethum, a variant (influenced by Greek lēthē ‘forgetfulness’), of letum ‘death’.
wiktionary
Learned borrowing from Latin lētālis(“mortal, deadly”), improperly written lēthālis, from lētum(“death”), improperly written as lēthum, as associated with Ancient Greek λήθη(lḗthē, “forgetfulness”).
Abbreviation of “ lauric acid ethereal salt”, so called because it occurs in the ethereal salt of lauric acid.
etymonline
lethal (adj.)
"causing or resulting in death," 1580s, from Late Latin lethalis, alteration of Latin letalis "deadly, fatal," from lethum/letum "death," a word of uncertain origin. According to de Vaan, from Proto-Italic *leto-, which is perhaps a noun from a PIE past participle of a verb meaning "let, let go," on the notion of death as "a letting go." If so, related to Old Church Slavonic leto "summer, year" (from notion of "going"), Russian leto "summer," (pl.) "age, years;" Russian let' (archaic) "it is possible, allowed;" Old Norse lað, Old English læð "land," Gothic unleds "poor."
The form altered in Late Latin by association with lethes hydor "water of oblivion" in Hades in Greek mythology, from Greek lethe "forgetfulness" (see Lethe).
