It started because Baby Boomers legit fucked the country's economy up, but they're the ones that are like "pull up your boot straps, I bought a house, a car, and paid for college out of my pocket" when they don't realize in their day it was possible. So Gen X, Gen Y (millenials), and Gen Z basically give them shit and tell them to shut up cause they're the ones that fucked up the economy.
But now retarded young people turned it into a lame ass meme where they say "ok boomer" to anybody that's even slightly older than them. For example:
I be like: "Takeshi 69 makes trash music"
Dumbass 19 year old is like: "ok boomer"