The story
From its facetious treatment as a time zone.
Etymology from Wiktionary, CC BY-SA 4.0.
Today it means: A notional system of time (or time zone) which others sometimes derogatorily ascribe to African Americans, and which they sometimes jocularly ascribe to themselves, to account for their supposed tendency to be tardy. Full definition →
From its facetious treatment as a time zone.
Etymology from Wiktionary, CC BY-SA 4.0.