Codswallop is nonsense or silliness. If you're hoping for an A on your paper, it's disappointing to get a D and a note from your teacher saying, "This is a bunch of codswallop!"
Codswallop is a colorful, informal way to describe something that's utterly ridiculous, especially a written or spoken statement you find absurd. A movie critic might describe a big-budget monstrosity as codswallop, and a haughty older brother might dismiss everything his younger sister says as "Nothing but codswallop." The word is primarily British, and it's been around since about 1960. Its origin isn't certain, but it may come from wallop, slang for "beer."