"boomer" is derogatory slang that started out as likening someone to a generational baby boomer, but what it means now is quite a bit harder to put into words.

If you want proof it's a thing, see https://en.wikipedia.org/wiki/OK_boomer. That's a pretty wretched source tho. It's written by and for the kind of people who think citing thinkpieces in The Guardian can have any relation to how slang is actually used in real life.