Box = bocks = "male goat's" = "the devil's".
The structure is often like this: "may the devil's something cause you harm".
One other thing that might be relevant for us is that the spelling with x (pox or box) appears more frequent when the devil is intended rather than the animal.
Edit: as Bernd wrote, in cases like "box" five wounds, it looks like God is meant rather than his counterpart with the cloven hooves. Still it looks like the expletive leans more towards x-use than the ingredient does.
The structure is often like this: "may the devil's something cause you harm".
One other thing that might be relevant for us is that the spelling with x (pox or box) appears more frequent when the devil is intended rather than the animal.
Edit: as Bernd wrote, in cases like "box" five wounds, it looks like God is meant rather than his counterpart with the cloven hooves. Still it looks like the expletive leans more towards x-use than the ingredient does.