Origin and history of literally

late 14c., "taking words in their natural meaning" (originally in reference to Scripture and opposed to mystical or allegorical), from Late Latin literalis/litteralis "of or belonging to letters or writing," from Latin litera/littera "letter, alphabetic sign; literature, books" (see letter (n.1)). Related: Literalness.

Meaning "of or pertaining to alphabetic letters" is from late 14c. Meaning "concerned with letters and learning, learned, scholarly" is from mid-15c. Sense of "verbally exact, according to the letter of verbal expression" is attested from 1590s, as is application to the primary sense of a word or passage. Literal-minded is attested from 1791.

common adverbial suffix, forming, from adjectives, adverbs signifying "in a manner denoted by" the adjective; Middle English -li, from Old English -lice, from Proto-Germanic *-liko- (source also of Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko). See -ly (1). It is cognate with lich, and identical with like (adj.).

Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.