Some folks at work objected to my wearing one inside. I didn’t care to fight it, but it did make me idly wonder if a similar cultural restriction exists in, say, Europe. Apparently doing it is frowned upon if you are not in the military.
Rural Slovenia is exactly the same as, say, London.
From here:
Etiquette in Europe
The US military has specific rules about when and where one wears a hat, and you must follow those rules exactly.
Failure to follow hat protocol is not treated harshly, but rather with pity. Failure to wear a hat when required is like not having pants on when expected.
Hmm. My workmates more seemed to say that I couldn’t wear one indoors UNLESS I was in the service, if that’s not what you’re saying. And I apolgize, Baron, for generalizing Europe as I did.
What kind of hat? I wouldn’t wear a fedora or a homburg indoors, but would certainly wear a “cowboy hat” or a fez.
Yes, I do own a fez.
A baseball cap. Though a fez would be spiffy.
Fezzes are cool.
In the military you don’t wear hats indoors unless underarms or your job requires it. Having been in the military doesn’t excuse you from manners.
“Cowboys and Jews have a common bond. They are the only two groups to wear their hats indoors and attach a certain importance to it.”
Kinky Friedman.
Under what conditions do your underarms require you to wear hats? 
Don’t worry - Barons tend to get uppity about little things like that :).
Certainly against traditional etiquette to wear any kind of hat indoors in the UK (at least for men - women in church for weddings often wear hats, though even that is much less common nowadays), but of course it depends on the situation. In the grill room at the Connaught, the waiter might specifically ask you to remove your bowler if you left it on, but no-one would bat an eyelid at you wearing your baseball cap in Tesco’s.
From my experience (I’m German who spent a few years in the UK, and also likes to travel), the rule not to wear hats indoors exists on the European continent as well but is not as strong a no-go as it is in the Anglo-Saxon world, be it the United States or the UK and the rest of the Commonwealth. This may, of course, be a simple result of the fact that there are, in general, fewer hat-wearing men on the continent than in the Anglo-Saxon countries.
Note that the taboo applies to men only, though. It’s perfectly acceptable for women to wear hats indooes, even in churches. I have read somewhere that these rules, like many fashion rules, date back to the days when wearing a hat would have been considered a preparation for fighting (putting on your helmet) and thus a threat when done indoors. Of course this may be folk history, like so many other attempts at explaining fashion and etiquette, but it sounds plausible and explains why the rule does indeed not apply to women.
Isn’t this pretty much a Christian thing?
From a restriction for church and worship to a general restriction on indoor hat use. Or does anyone have contradictory evidence?
And on a tangent, why does the pope get to wear a hat?
The obligation (not permission!) for women to wear a head covering in church is indeed from Corinthians, but also one of the things that got corrupted pretty much from the get-go (can’t show my hair for modesty? here come lace and embroidery!) and one which got removed by Vatican II.
As for general etiquette, usually in Spain wearing a non-work-related head covering indoors is frowned upon.
Gotta love the Kinkster.
Its a Western culture thing. Its gender based - for probably a number or reasons (women’s hats required pins and therefore couldn’t be taken on and off in addition to the modesty of covering your hair that is VERY old), and you get to wear them for religious reasons (Jews and Sikhs) without offending etiquette. Being a Yankee’s fan is not religious, no matter what borough you are from.
Its really starting to fall away - people wear baseball caps in homes and in public in America - but it isn’t something that will leave a good impression at work or with your friend’s grandparents.
Israelis, being both Jews *and *cowboys, wear their hats wherever they want.
Take a look at old movies. Every man wore a hat in those days. Not doing so was very lower class. It was still the custom to take hats off when going indoors but the cool kids - like reporters and private eyes - kept their hats on as a way of signaling that they were above such middle-class mores.
Or, more likely, to help identify them to the viewer.