The origins of XXX as FIXME

The token XXX is frequently used in source code comments as a way of marking some code as needing attention. (Similar to a FIXME or TODO, though at least to me XXX signals something far to the hacky end of the spectrum, and perhaps even outright broken). It's a bit of an odd and non-obvious string though, unlike FIXME and TODO. Where did this convention come from? I did a little bit of light softw