He kept saying lockbox because there was (and still is) a rumor that Congress has been stealing social security to pay for wars and that's why it's bankrupt.

The reality is that the money is invested in US bonds. Although technically Congress uses that money for wars and other stuff, they pay it back at interest which is overall good for social security (at least compared to cash just sitting there). I suppose if the money was invested instead of being used to buy extremely low yield bonds it could have grown many times over by now, so that's "stealing to pay for wars" by SOME definitions.... But it's not what most people mean with their accusations.

Tl;dr: it's in a lockbox!