1580s, "pertaining to or constituting a lengthy heroic poem," via French épique or directly from Latin epicus, from Greek epikos, from epos "a word; a tale, story; promise, prophecy, proverb; poetry in heroic verse" (from PIE root *wekw- "to speak").
Extended sense of "grand, heroic" is recorded in English by 1731. From 1706 as a noun in reference to an epic poem, "A long narrative told on a grand scale of time and place, featuring a larger-than-life protagonist and heroic actions" [Miller Williams, "Patterns of Poetry"]. Earlier as "an epic poet" (1630s).
I believe the word 'epic' is usually understood by English readers to mean merely a long and grand poem instead of a short slight one—at least, I know that as a boy I remained long under that impression myself. It really means a poem in which story-telling, and philosophical reflection as its accompaniment, take the place of dramatic action, and impulsive song. [Ruskin, "Elements of English Prosody, for use in St. George's Schools," 1880]
It forms all or part of: advocate; avocation; calliope; convocation; epic; equivocal; equivocation; evoke; invoke; provoke; revoke; univocal; vocabulary; vocal; vocation; vocative; vociferate; vociferous; voice; vouch; vox; vowel.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit vakti "speaks, says," vacas- "word;" Avestan vac- "speak, say;" Greek eipon (aorist) "spoke, said," epos "word;" Latin vocare "to call," vox "voice, sound, utterance, language, word;" Old Prussian wackis "cry;" German er-wähnen "to mention."
1540s, "epic poem," also "a book of an epic" (suitable for recitation at one time), from French rhapsodie, from Latin rhapsodia, from Greek rhapsōidia "verse composition, recitation of epic poetry; a book, a lay, a canto," from rhapsōdos "reciter of epic poems," literally "one who stitches or strings songs together," from stem of rhaptein "to stitch, sew, weave" (from PIE root *wer- (2) "to turn, bend") + ōidē "song" (see ode).
According to Beekes, the notion in the Greek word is "originally 'who sews a poem together', referring to the uninterrupted sequence of epic verses as opposed to the strophic compositions of lyrics." William Mure ["Language and Literature of Antient Greece," 1850] writes that the Homeric rhapsōidia "originally applied to the portions of the poems habitually allotted to different performers in the order of recital, afterwards transferred to the twenty-four books, or cantos, into which each work was permanently divided by the Alexandrian grammarians."
The word had various specific or extended senses 16c.-17c., mostly now obsolete or archaic. Among them was "miscellaneous collection, confused mass (of things)," thus "literary work consisting of miscellaneous or disconnected pieces, a rambling composition." This, now obsolete, might be the path of the word to the meaning "an exalted or exaggeratedly enthusiastic expression of sentiment or feeling, speech or writing with more enthusiasm than accuracy or logical connection of ideas" (1630s). The meaning "sprightly musical composition" is recorded by 1850s.
"a song, a short, simple poem intended to be sung," c. 1600, from French chanson, from Old French chançon "song, epic poem" (12c.), from Latin cantionem (nominative cantio) "song," from past participle stem of canere "to sing" (from PIE root *kan- "to sing").
"harsh-sounding steam-whistle keyboard organ," 1858, named incongruously for Calliope, ninth and chief muse, presiding over eloquence and epic poetry, Latinized from Greek Kalliope, literally "beauty of voice," from kalli-, combining form of kallos "beauty" (see Callisto) + opos (genitive of *ops) "voice" (from PIE root *wekw- "to speak").
c. 1600, "Odyssey," title given to one of the two great epic poems of ancient Greece, from Latin Odyssea, from Greek Odysseia, the ancient name of the Homeric poem telling tales of the ten-year wanderings of Odysseus, king of Ithaca, seeking home after the fall of Troy. Figurative sense of "long, adventurous journey" is recorded by 1889.