Definitions include: It means something that looks too good for words. Refers to clothing, automobiles, etc... Could be a synonym for the slang word fly, but more intense. Originated on the streets of New York City.
Definitions include: "Milkshake is just that thing that makes a woman stand out from everyone else. It's a thing that makes you sensual and warm and maternal. It could be about breasts but I don't have huge tits so you gotta work with what you got." - Kelis