...yo no entiendo ni papa.
[Caption 53, Si-Sé > entrevista > Part 1]
When Carol C. of Si*Sé says with a shrug, yo no entiendo ni papa, it's easy enough for us to understand by the context that she doesn't understand a word. She could also have said no entiendo nada, which means "I don't understand anything." [Remember: you use the word nada ("nothing") instead of algo ("anything") after no in negative expressions in Spanish.]
But here singer C.C. chooses a common Spanish phrase for emphasis -ni papa. Ni means "not even" or "nor." That much is straightforward. But papa is one of those words with an almost comic array of meanings -from "Pope," as in más papista que el papa ("more papist than the Pope"), to "potato," as in papas fritas ("french fries"). Well, one of the many meanings of papa comes from the Latin "pappa" and it means "baby food," "mush," or "pulp." And that's the meaning most commonly associated with the phrase ni papa (literally: "not even mush").
No puedo ver ni papa
"I can't see a thing."
Él no sabe ni papa
"He doesn't know a thing."
Es una papa.
It's a piece of cake. [It's easily done/easily accomplished.]
No te preocupes por el examen, es una papa.
Don´t worry about the exam, it´s a piece of cake.