The story
Blend of June + nineteenth, commemorating the anniversary of June 19, 1865, when Union Army General Gordon Granger (1821–1876) issued General Order No. 3 in Galveston, Texas, to enforce President Abraham Lincoln’s Emancipation Proclamation of September 22, 1862. The General Order stated that all previously enslaved people in Texas were now free. The word entered wider usage after becoming a federal holiday in 2021.
Etymology from Wiktionary, CC BY-SA 4.0.