這種問題建議你找字源學(Etymology)的字典，以下是我幫你找到的結果：
popular name of a troublesome, voracious insect genus, 1620s, folk etymology
(as if from cock (n.1) + roach; compare cockchafer) of Spanish cucaracha
"chafer, beetle," from cuca "kind of caterpillar." Folk etymology also holds
that the first element is from caca "excrement," perhaps because of the
insect's offensive smell.
翻譯：
Cockroach是一種麻煩的、貪食的昆蟲，來源自1620年代的民間傳說。這個字由cock和
roach組成，可以與cockchafer（金龜子）一起做對照，cock可能來源自cuca（一種毛蟲
），民間字源傳說聲稱，cock其實原本是caca代表糞便，有可能是因為蟑螂的惡臭味。
總結，我覺得caca還蠻有趣的，caca真的是跟小朋友講『便便』、『摁摁』、『臭臭』的
那種感覺。