这种问题建议你找字源学(Etymology)的字典，以下是我帮你找到的结果：
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.
翻译：
Cockroach是一种麻烦的、贪食的昆虫，来源自1620年代的民间传说。这个字由cock和
roach组成，可以与cockchafer（金龟子）一起做对照，cock可能来源自cuca（一种毛虫
），民间字源传说声称，cock其实原本是caca代表粪便，有可能是因为蟑螂的恶臭味。
总结，我觉得caca还蛮有趣的，caca真的是跟小朋友讲‘便便’、‘摁摁’、‘臭臭’的
那种感觉。