A milkshake is a black person who pretends as if they is white. It's as if they are almost entirely made up of vanilla ice cream, but have just a littleeeeee bit of chocolate syrup that makes them a different color.
Brandon: "I love going to New Kids on the Blocks Concerts!" (as he's dancing and singing)
Me: "Wow, Brandon. You are such a milkshake! You act whiter than me and I'm 100% c-a-u-c-a-s-i-a-n."