Most commonly used with buttload, assload, or shitload.
The modifier 'metric' has been co-opted from the term 'metric ton,' which is 205 pounds more than a 'ton.'
So, an 'assload' is a lot, but a 'metric assload' is even more.
Paul: "Yea, they suck! Listen to some good music like Green Day, John Mayer, or Linkin Park!"
Randy then proceeds to push Paul's lifeless body into the Hudson.
Probably drawn from the idea that the metric system is "better" than other systems.
Lil Wayne: Yo dawggg, that shit was metric AF.
2. An adjective that describes something as making more sense, better, and/or more awesome than any other possibility.