Poppycock means nonsense, but with a subtle connotation not usually mentioned in definitions, namely that its use often suggests willfulness. For instance, a spelling mistake is unlikely to be described as poppycock, but a false or controversial statement could certainly be termed such.
Poppycock, for me at least, conjures up images of outraged, red-faced British ex-officer types, so I was very surprised to learn that it is actually American in origin. Its use comes from Dutch settlers, and is derived from their word pappekak, which means exactly what it sounds like: soft shit.