It means OLD... It's a generation that's called the " Baby Boomers" but in slang if you say boomer it means old. So basically if you call someone/something a boomer then you are calling them old or calling the thing old.
Bob: My favorite artist is Elvis!!
John: Man {WTF} THAT DUDE IS A BOOMER!!
An act in which a person grabs the sides of a metal folding chair tightly, pulling the ass as close to the seat as possible, and then farts as forcefully as possible.
The boomer reverberates loudly in all directions and resonates through the chair.
Tom: "Holy crap! Did you hear that?"
John: "Yeah, what was that, a gunshot?"
Tom: "Naw, Vinnie just did a boomer!"