A tidy way to call something total nonsense or made-up rubbish. It sounds a bit old-fashioned and slightly posh, which makes it even funnier when you drop it in an argument. The origin’s not nailed down, but people link it to wallop meaning cheap booze and cod meaning fake. Use it when someone’s chatting absolute fantasy.
"He reckons he used to play pro football. Mate, that’s codswallop. Saw him at five-a-side last week, he couldn’t hit a barn door."