Poppycock is the brand name of a confection made from popcorn and nuts, covered with a caramel glaze, somewhat like Cracker Jack.
In Dutch, "pappekak" means "soft shit", i.e. diarrhea.
I suppose "pappekak" or "poppycock" originally was exactly like "bullshit" today, that is, it had both a literal meaning and the figurative meaning of "foolish talk, nonsense."
Since most Americans didn't know the literal meaning, it could be printed at a time when "bullshit" could never be printed.
An early example of the word "poppycock" in print comes from the works of the American dialect-humorist "Artemus Ward."
"I venture to say that if you sarch the earth all over with a ten-hoss power mikriscope, you won't be able to find such another pack of poppycock* gabblers as the present Congress of the United States of America."
The British edition of this book added an explanatory footnote which was absent in the American edition:
*"All poppycock!" Anglicè, all sound and fury, signifyin nothing.