The terminology was coined by a British MP.
Chlöe Swabrick, who belongs to the Green Party, silenced a fellow member of the U.K. Parliament by saying, "OK, boomer" – a phrase adopted by millennials to call out people from the baby boomer generation.
From Urban dictionary...
When a baby boomer says some dumb shit and you can't even begin to explain why he's wrong because that would be deconstructing decades of misinformation and ignorance so you just brush it off and say okay.