I can guess it probably means "what's happening", or "what's up". Am I right? I am curious where this phrase comes from and how to use it properly.
- (slang) what is happening?
The phrase is Afro-American in origin, from the 1950s. From 1970's Black Slang: a Dictionary of Afro-American talk By Clarence Major:
Shaking: (1950's) same as Happening; example, "What's shaking?"
Here's some published uses from the 1950s.
From Sig Byrd's Houston by Sigman Byrd, 1955:
There's also this from 1931, it could be related but it's probably not:
"They've found you on the beach, that's what you hear, what's shaking your shoulder. Your heart's still beating. You've got time to go back, to live, to find someone else than Sue. Sue's meeting John on Thursday. Go back to the beach.
Anyway, as to how to use What's shaking?, the excellently titled Flappers 2 Rappers: American Youth Slang (1996) gives some variations:
Shaking or swinging is indeed what certain parts of the adult body can do. As a native speaker and city dweller, I can say that by intonation, that reference is the etymology. Similar to "How's your sorry ass been?" of "How's it hangin'?"
Do not use the expression with your grandparents, but only with peers.