Revan: "You just called me a meatbag again!"
HK-47: "Explanation: It's just that... you have all these squishy parts, master. And all that water! How the constant sloshing doesn't drive you mad, I have no idea."
Revan: Come to think of it, neither do I.
The adjective came into useage after the droid referred to an apprentice of Darth Revan as such. Finding it highly amusing, the Dark Lord permanently programmed it into the droid, providing a basis for many comical lines throughout the game.
The word is meant to be used derogatorily. It should be kept in mind that this word is restricted to robotic verbage, and the effect, when uttered by a human, is negated to the point of "nerdiness".
The term "meat-bag" means one who is composed of organic parts. The implication being that, as sensitive and fragile organic beings, we cannot simply exchange our entire inner-workings easily, nor preserve them until rust consumes them.