mid-15c., "causing fear," from Old French formidable (15c.), from Latin formidabilis "causing fear, terrible," from formidare "to fear," from formido "fearfulness, fear, terror, dread." Sense has softened somewhat over time, in the direction of "so great (in strength, size, etc.) as to discourage effort." Related: Formidably.
late 14c., "characterized by exaggerated self-importance or an ostentatiously dignified style," from Old French pompos (14c., Modern French pompeux) and directly from Late Latin pomposus "stately, pompous," from Latin pompa "pomp" (see pomp). More literal (but less common) meaning "characterized by magnificence and dignity" is attested from early 15c. In 15c. it also could mean "fierce, formidable." Related: Pompously; pompousness.
late 14c., of persons, "worthy of honor, venerable" (a sense now obsolete); late 15c., "that is to be dreaded or feared, formidable, terrible," also often "valiant," from Old French redoutable (12c.), from redouter "to dread," from re-, intensive prefix, + douter "be afraid of" (see doubt (v.)).
The verb also was in Middle English, redouten, "to fear, dread; stand in awe or apprehension of; honor" (late 14c., from Old French) and was used through 19c., though OED marks it "now rhetorical."
c. 1200, "great in quantity or extent" (also "great in size, big, large," a sense now obsolete), a worn-down form (by loss of unaccented last syllable) of Middle English muchel "large, tall; many, in a large amount; great, formidable," from Old English micel "great in amount or extent," from Proto-Germanic *mekilaz, from PIE root *meg- "great."
As a noun, "a large quantity, a great deal," and as an adverb, "in a great degree, intensely, extensively," from c. 1200. Since 17c. the adverb has been much-used as a prefix to participial forms to make compound adjectives. For vowel evolution, see bury. Too much was used from late 14c. in the senses "astonishing, incredible," also "too offensive, unforgivable." Much-what "various things, this and that" (late 14c.) was "Very common in the 17th c." [OED] and turns up in an 1899 book of Virginia folk-speech as well as "Ulysses."