When somebody says "Ok, Boomer" it's like someone saying, for example, "That's old news" or "that was a long time ago and you're just learning about this now."
Guy 1: I just learned what selfies are!
Guy 2: What are they?
Guy 1: They're like photos you take of yourself!
Guy 2: Ok, Boomer.
Guy 2: What are they?
Guy 1: They're like photos you take of yourself!
Guy 2: Ok, Boomer.
by Mr. Stevens's class January 14, 2020