I see the word milkshake used now and then my youth, but I have know idea what it means. Maybe comes from a song? The context is often such that it makes me think it refers to body.
Could you give an example of the sort of usage that makes you think it means “body”? I’ve only ever heard it used to refer to a milk-and-ice-cream beverage.
From the song “Milkshake,” by Kelis:
If an image of bobbling breasts comes to mind, I’m think you’re looking in the right direction.
You know, I always like to imagine this song is about a woman who makes really good milkshakes . . . good quality ice cream, whole milk, the works. A really good milkshake most certainly would bring THIS boy to the yard . . . mmm . . .
You mean it isn’t? 
My (19 year old suburban white) interpretation of the song was that the “milkshake” was a guy’s cum. That is, that Kelis, the singer of the song “Milkshake”, was bragging about her skill at giving blowjobs.
But then again the “urbandictionary.com” link probably has a more authorative definition if you synthesize all those people’s interpretations.
This from UrbanDictionary.com: A girl’s milkshake… is “the way she carries her shit”… her walk, her ass, her body, whatever makes her hott… her "stuff.
This is how I’ve always thought of it (since the song came out, that is). How can you teach someone to have big boobs?
“It’s not the size, but how you use it” applies to women as well as men. 
from site;
If you ever heard Nelly’s “Pimpjuice” he talks about different things girls are attracted to or whatever you use to get women. “Milkshake” is basically the same thing. Nothing in particular.
(although the way she uses it in the song suggests some kind of physical act. not an object)
Or that je ne se quio of women.