Boomer is short for Baby-boomer. Not necessarily pertaining to their actual age, but a mentality from a time where the finite value of the world was not properly appreciated; a society whose capitalistic ploys has led to a mass ecological/economical crisis; someone not very conscious about preserving the rest of the world. One who thinks along the lines of "oh, my carpet outside of my fireplace is on fire, and burning naturally happens, so i may as well poor gasoline on it instead of water, cause f#$@ the prospect of preserving a home for my family to live in. Oh and I just spawned 10 grandchildren while we had this talk" Except this analogy doesn't even begin to depict the actual crisis, as there is only one home available to the entire family for all time. (Unless space-X takes off, but to rely on that solely would be dumb, especially if you're not super rich).
Boomer :"Maybe I would listen to Greta Thurnberg if she wasn't so rude about asking us to not poor gasoline on the house."
Proper response- "Ok, Boomer"