Milkshake or milkshakes refers to a women's breasts, aka titties
They are capable of producing milk while a woman is breastfeeding, and are known to "shake" or bounce.
In "Beauty Shop" with Queen Latifah from 2005, a boy named Willy comes in her beauty shop selling chocolate. When he first sees Queen Latifa (whose character is named Gina) he admiringly says to her, "How you doing? Damn you a thick one! You got top, AND bottom!". Gina walks away from him after telling him he will need to learn to be "a little more courteous to the ladies" if he wants to sell chocolate there. "Yeah I got that no doubt"" he says. As Gina walks away he says, "Yo Gina! You gonna buy some 5 dolla chocolate to go with those healthy milkshakes you got?". Gina's mouth drops and we next see Willy and his chocolates getting tossed out of the shop on to the sidewalk.
Although I'm fairly certain the word was likely in common colloquial usage at that time and place, I believe this is the first and possibly only notable reference to "milkshake" as slang until the pop song "Milkshake" by Kelis on her album "Tasty" from 2003 popularized the word.
According to Kelis the word in her song is used to mean "something that makes a woman special". ( aka titties?)
The song was featured in the 2004 film "Mean Girls" and Heather Phares of Allmusic said it is, "basically a three minute tutorial in sex appeal".
"Yo Gina! You gonna buy some 5 dolla chocolates to go with those healthy milkshakes you got?"