Via Hellogiggles.com:

There’s no doubt that in the adrenaline-rush of watching Star Wars: The Force Awakens, you could easily miss some of the more juicy details, but one fact that stuck out to fans everywhere was that C-3PO had a red arm that was NEVER EXPLAINED.

But, knowing that we were dying to know why oh why C-3PO had that new appendage, Disney has finally given the reason behind it. A recently released one-shot Marvel comic (written by James Robinson, Tony Harris, and Joe Caramagna) tells the story of our brave C-3PO (aka Threepio) on a mission for the Resistance with a bunch of other droids. During the dangerous mission, things go awry and all of the droids lose their lives except our favorite little protocol partner and one other red droid named Omri. While Omri, like the others, eventually sacrifices himself for the greater good of the Resistance and their mission, Threepio only ends up losing his arm. In an effort to remember the sacrifices of the brave droids and the lessons he learned while serving with them (a feeling he’s capable of thanks to his advanced sentient programming), he decides to reassemble himself with Omri’s red arm.

Basically, he uses the limb in remembrance of his brave fallen comrades and the sacrifices they and so many others have made for this greater good. With heavy memories like that, it’s no wonder he loves having the goofy R2-D2 around to provide a little levity with his courage.

The story is a reminder that even a polite droid can have a haunted past. So be kind to your robot neighbors and friends. They, obviously, can feel all the feelings, too.