Como dizer "Ideia de jerico" em inglês

Also hare-brainded idea. To mean "idéia descabida/treslocada/idiota/estúpida/maluca" etc...

By the way, the "harebrained idea" (or bird-brained idea - if you wish) thing can also be expressed in ironic ways.

Think of me stating something and the other person thinks the idea is idiotic/ridiculous, then he/she could say "That was a brilliant idea!" or "what a brilliant idea!"

That is, it was said to mean the opposite. But it goes with the body language, tone of voice, in jest, etc...
It's just a passing comment, we know that "jerico" (a donkey) would think of irony! LOL

On the other hand, there are more to to language than meets the eye, we can think of a feather-brained idea when a boss say to us "If you come up with another brilliant idea like that, you’re fired. "
Also in indirect ways "I don't think it's not a good idea/I think it's a bad idea" if just have proposed to bungee jump using a normal wire rope...

Another indirect translation I found was:
What on earth were you thinking?
Que ideia de jerico é essa?
Source:Reverso Context

Cockamamie idea

Exemplo: I say this with great respect to them but they’ve failed. Last year, Gordon Ramsay had some cockamamie idea.https://www.eadt.co.uk/news/national/uk ... w-rip-off/