Summary
Fallout: London, a fan game built usingFallout 4, has added a rather unique method of keeping your rad count down. As some players have discovered, the “Jimmy Hat” item actually increases your resistance to radiation, somehow.
And yes, Jimmy Hats are exactly what you think they are - hats for your Jimmy. Otherwise known as johnnies, raincoats, rubbers, or, you know, just condoms. Somehow, popping one of these on decreases the amount of radiation your character builds up. Incredible stuff.

Fallout: London Adds A Unique Way Of Increasing Radiation Resistance
Shout out to Reddit user CWalkthroughs for spotting this neat little feature. I guess it’s keeping… part of you safe from being exposed to nuclear fallout, so that could explain why it’s so effective against radiation.
As highlighted by Reddit user RelativeWeekend453, these were also an item inFallout 2, so it’s not just thrown in there because it sounds like something a Brit would call a condom. However, in Fallout 2, they behaved likeactualcondoms and were there to stop you from getting another character pregnant if you slept with them. Don’t worry - futureFalloutgames didn’t make you worry about that.
This is far from the only reference that Fallout: London players have found. As we covered earlier today,there’s a whole side quest that revolves around Doctor Who, tasking you with tracking down 15 TARDIS' - or as the game calls them, “Unusual Call Boxes”. Much like the Jimmy Hats, this could also be a throwback to classic Fallout, as you could stumble across a TARDIS in the first game too - where it was also called an “Unusual Call Box”.
If you want to try Fallout: London yourself, it’s free onGOG. You’ll need to downgrade your version of Fallout 4 to the pre-next-gen update build to get it to work.
Fallout 4
WHERE TO PLAY
In a region known as The Commonwealth, the sole survivor of Vault 111 must search for their missing child. Fallout 4 adds more to the series, including base building and an enhanced armor system, as well as an open world to traverse as you please.