Balderdash is something said or written that doesn't make sense, such as a ridiculous claim or a bad argument. In other words, it's complete and utter nonsense.
Balderdash is a fun, somewhat old-fashioned term for "nonsense" or "rubbish." It carries a British, slightly theatrical overtone, and is best used in a light-hearted way for trivial matters. Calling someone's argument "balderdash" feels more like a playful rebuke than a harsh insult. Save weightier terms like fallacy or misinformation for important or professional disputes, as serious concerns deserve a more serious word than balderdash.