"excrement," a nursery word but a very ancient one (PIE *kakka-), forming the base word for "excrement, to void excrement" in many Indo-European languages, such as 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. The word in this form appears in English slang c. 1870, and could have been taken from any or several of the languages that used it (Spanish, Modern Greek).
