From Winter Pasture, by 李娟 (Astra Publishing, 2021), Kindle p. 161:
Of course, there were occasions when the siblings argued. Sometimes they even insulted each other in Mandarin. Sister would shout, “Ben dan!” (idiot) and brother would shout back, “Wang ba dan!” (bastard)—which amused them to no end.…
Then, they both turned to ask me, what do “ben dan” and “wang ba dan” mean? For the sake of complete honesty, I offered them a boring literal explanation: a ben dan is a chicken egg that’s gone bad. And a wang ba dan … luckily, I had just seen “The Tortoise and the Hare” in Nurgün’s Kazakh textbook, so I pointed to the tortoise: “This is wang ba.” They let out an “oh.” Then I added, “Wang ba dan is its child.” A disappointed “oh”; they were unable to understand what was so special about a bad egg and a tortoise’s child that these terms could be used as insults? Thoroughly underwhelmed, that was the last time they insulted each other using these words.
My ABC Chinese-English Dictionary (U. Hawaii Press, 1996) defines 王八 wángba as 1. tortoise, 2. cuckold, 3. son-of-a-bitch. It next lists two expressions flagged as colloquial: 王八蛋 wángbadàn N. turtle’s egg, son-of-a-bitch; and 王八羔子 wángba gāozi N. baby turtle, son-of-a-bitch.
The same source defines 笨 bèn as ADJ. 1. stupid, dull; 2. clumsy, awkward; 3. cumbersome. Lower on the same page it lists 笨蛋 bèndàn N. fool, idiot.