[Y_Y]

For the benefit of others who aren't so familiar couillons, it's one of the many proud descendants of the Latin cōleus which means "balls" in the sense of testicles or courage. Other etymological cousins include the Catalan colló, the Portuguese colhão‎, and of course the notorious Spanish cojón.

Hardly surprising these are still popular idioms.

[beAbU]

Still not sure what the phrase means tbh.

A direct google translation comes back with "disabuse you idiot"

So maybe it means "idiot proof"?

[auxym]

Yes, idiot proof would be a correct translation. Or more literally, since "détrompe couillon" is a noun, an "idiot proofer", or "idiot-proofing feature".

[beAbU]

I see, thanks.

So "couillon" is a synonym for balls and idiot if I'm understanding the GP's references correctly.

[liotier]

I guess I should jump back in and clarify what I left unexplained when starting the thread.

Contrary to "couilles", which are indeed "bollocks", "couillon" has no exact english synonym. Closest I can imagine, in spirit, would be "bollocks-brained", or "nutcase" which also carries the anatomical undertone.

So, "un détrompe-couillon" could directly translate as "a foolproofer" - though that translation is tamer than the original for lack of testicles.