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.