Poppycock is such a satisfying word to say. It's staccato syllables sound simultaneously childish and dirty, a prime combination for humor.
It sounds like popcorn popping, but with that cock at the end, it's popcorn you wouldn't want to eat.
Mark Peters, in his new book Bullshit: A Lexicon, tells us that the word comes from the Dutch word poppekak, which appears in the idiomatic phrase zo fijn als gemalen poppekak, "as fine as powdered doll excrement."
Poppycock is literally doll poop.
Barring the creation of an Irritable Bowel Syndrome Barbie, poppycock is a fantastical substance you aren't likely to find in the real world, which makes it a perfect word for describing something that coudn't possibly be true.
Plus, as I mentioned, it's just fun to say.
And, unlike bullshit, you can use it in church in front of your grandmother without getting the stink eye from other parishioners.