Meatwad is a ball of meat, capable of transforming into anything.
The Leftover Meatball
One day, a full Mario was eating spaghetti, but he got stuffed, and as so, a meatball fell out, and rolled out of Mario's house. The meatball rolled through town, and because no one noticed it, the meatball aged, eventually to the point to where he could talk.
Aqua Teen Hunger Force
As said above, Meatwad will listen to what anyone says, including Master Shake and Carl, who may be bad influences on Meatwad. He is always tortured by Master Shake, but his blows bounce off of Meatwad's body. Meatwad has been tricked into helping shake steal things, like Kirby's pie.