Back in 1963, when the term “baby-boomer” was first published, it carried just one definition: a person born during the tail end or in the decade after World War II, when optimism of the world caused a tremendous spike in births. Frequently shortened to “boomer,” over the years the phrase has been imbued with layers of meaning and implication. Over the years “boomer” has become a blanket term referencing the predominant trends, values and concerns of an entire generation.
“Boomer” is also, evidently, an insult to an older person. The development of the word as a pejorative is a pretty recent phenomenon, best exemplified by and on social media and other internet platforms. “OK boomer” has become a derisive repudiation, a bit mocking in tone, like a verbal eye roll and it’s directed specifically by a young person toward someone older.
On a personal basis, I couldn't care less!