"trivial talk, nonsense," 1865, American English, probably from Dutch dialect pappekak, from Middle Dutch pappe "soft food" (see pap) + kak "dung," from Latin cacare "to excrete" (from PIE root *kakka- "to defecate").
"soft food for infants, gruel, porridge," late 14c., from Old French pape "watered gruel" and Medieval Latin papo, both from Latin pappa, a widespread word in children's language for "food" (compare Middle High German and Dutch pap, German Pappe, Spanish, Portuguese papa, Italian pappa), imitative of an infant's noise when hungry; possibly associated with pap (n.2). Meaning "over-simplified idea" first recorded 1540s.
It is the hypothetical source of/evidence for its existence is provided by: Greek kakke "human excrement," Latin cacare, Irish caccaim, Serbo-Croatian kakati, Armenian k'akor; Old English cac-hus "latrine."
Etymologists dispute whether the modern Germanic words (Dutch kakken, Danish kakke, German kacken), are native cognates or student slang borrowed from Latin cacare. Caca appears in Modern English in slang c. 1870, and could have been taken from any or several of the languages that used it.