What is that fool put in the boot?

[englishhobby]

What does the question mean ('What is that fool put in the boot?') Could you rephrase it?
Here's the audio: http://lms.vspu.ru/files/2020/11/29.2-We-should-be-there-by-two.mp3
And here's the script:
- We should be there by two.
- Yes, it’s a full moon and the route’s good.
- I’d put the things in the boot.
- The boot’s full.
- What is that fool put in the boot?
- I wouldn’t look if I were you.
- Or should I say – who’s he put in the boot?

P.S. I understand that it's black humour, and still I can't make out what the question means.

 

[emsr2d2]

He says "What HAS that fool put in the boot?"

Also, he says "I'll put the things ...", not "I'd put ...".

There's no black humour there, other than at the very end I suppose, when one of them suggests that someone else might have put a person in the boot of the car.