Among all the words I know for "old person", this one is certainly the one I hear most often (I heard it just yesterday). Derived from the expression baby boomer (itself only first attested in 1963, and in its modern meaning in 1970), boomer itself is very recent: Know Your Meme dates "OK, boomer" back to 2015, with it really only ...
What’s cooking, good looking?
Kia Ora (New Zealand)
G'day mate (Australia)
Whazzzuuuuup (yes you have to drag it out)
What’s with the face? (if someone looks angry or sad)
Greetings earthling (funny/nerdy)
Note that "What's doing" (suggested by @Lambie) is NOT grammatical.
it just means to be arrested, because a cop would grab you by the collar sometimes, it doesn't have anything to do with taking the wrap for someone else's crime. not sure of the origination, but its just an old school way of saying being arrested.