I doubt the GitHub thread was "a long poem, typically one derived from ancient oral tradition, narrating the deeds and adventures of heroic or legendary figures or the past history of a nation"
The adjective is obviously. derived from the noun, and as such had much of the same meaning originally: "relating to or characteristic of an epic or epics: our national epic poem Beowulf."
And no, I'm not proud of having to weigh in on this subject.
Mine was OS X dictionary IIRC, but here's dictionary.com:
adjective, Also, epical
1.
noting or pertaining to a long poetic composition, usually centered upon a hero, in which a series of great achievements or events is narrated in elevated style:
Homer'sIliad is an epic poem.
2.
resembling or suggesting such poetry:
an epic novel on the founding of the country.
3.
heroic; majestic; impressively great:
the epic events of the war.
4.
of unusually great size or extent:
a crime wave of epic proportions.
5.
Slang. spectacular; very impressive; awesome:
Their burgers and fries are epic!
So if (4) is what you mean, it's not exactly the first definition for the adjective either.