Jank was a seperate term from junk. Junk was derived from the B/G/x (favoring White, and almost exclusively white after Jund appeared as a term) archetype where you just tossed strong cards together in a pile. Junk shares similar roots to Rock, although Rock is tied to a specific deck.
The earliest instance of "Jank" terminology colloquially applied to a magic deck was R/W back around the time of revised-6th ed, as you played a lot of cards that were individually unimpressive, and seemed to (and often did) have anti-synergy.
Swords to Plowshares in a deck full of Savannah Lions and Lightning Bolts with no form of card advantage? Was pretty much Heresy back then, since it went against both the "Card Advantage Uber Alles" school of thinking and the "Measure every card in how fast it kills them, any left-over cards in their hand when they die is card advantage for you" school.
This anti/lack-of -synergy aspect is often what people refer to when they call a deck Jank. When referring to individual cards, the anti-synergy descriptor tends to go out the door and the term defaults to a stand in for "seems pretty bad."
If I were to play a Big Rig Apex deck with 3 Apocalypses, I'd call it a Jank Deck.