Codswallop is a British informal word for nonsense: talk or ideas that are silly and not true. You can call a claim codswallop, or you can fire it back as a reply — “Codswallop!” It feels jokey, a little old-fashioned, the sort of word you hear from a grandparent or in a comedy, not in a science paper or a job interview. Though actually, nobody can pin down where it came from. It is clearly on record from a British TV comedy in 1959, and the writers said people already used it then. A later hunt for earlier uses did not turn up much. Some people guess it was built from old slang — a fake kind of beer — but that is only a guess, and the tidy story is tidier than the evidence. Spellings like cod's wallop came later; the older written form is one word.