The story
By analogy with OK, boomer.
Etymology from Wiktionary, CC BY-SA 4.0.
Today it means: Used to sardonically counter or dismiss a younger person (typically a Gen-Zer or millennial) as lacking an understanding of the circumstances, priorities, or culture of an older generation, especially by a baby boomer. Full definition →
By analogy with OK, boomer.
Etymology from Wiktionary, CC BY-SA 4.0.