What makes a word offensive?

Whilst I was sat on the bus yesterday, I overheard a group of teenagers discussing various things. As per the usual social requirement at that age, every 5th word was an expletive. Not exactly the best ambiance for my journey home, but it got me thinking.

What specifically about the words "fuck", "shit", "cunt", etc. makes them so vulgar? Clearly they represent negative things in society, but what makes those particular words so important? Also, what makes words with similar pronunciation characteristics (e.g. "crap" and "damn") so much milder?

I'm aware that there will be some complex etymology of each, but what I'm really interested in is the structure and formation of the word itself. Why aren't "dotard" or "popinjay" considered to be curses, when "fuck" is? At certain times they held similar sentiments.

Is there something specific about how we pronounce them that makes them superior in situations where cursing is required? I'd also be interested to know how this works in other languages, too.

It's just whatever society decides is offensive. Right now, culturally, sex is something of a taboo subject, and as such vulgar sexual words are usually offensive.

Hundreds of years ago, sex wasn't so taboo, and religious epithets were the main offensive language, such as "damn" or "God" or whatever, which is why you have ways around them from back then, like "Zounds" which means "God's wounds." Any time you see the weird Z in old stuff like that that sounds very old fashioned and a little silly now, it's usually a stand in for "God" in order to get away from how offensive it was to take the Lord's name in vain when they came about. Zounds is the only one I can really think of that's survived this long.

However, if you go back and read old stories from the 1300s or whenever, you'll find that they use the word "cunt" (or "cunte" or "coynte" or any of the other spellings they used then) a lot just in passing like it's nothing at all. Same with a lot of our offensive language, at least with the older words.

One commonality you'll often find with vulgar words is the presence of fricatives (the sh sound, for example) and hard consonants like 'k' and 't'.

You can hear the angry, frustrated release of emotion in words like fuck and shit, which both share the above characteristics.

Plosive sounds also work well, like the bursting p that makes crap a fine candidate -- it even feels like you're spitting when you say it.

When people use "minced oaths" as substitutes to the vulgar originals (fudge or shoot), you'll notice that the hard sounds are noticeably softened (the hard ck replaced by the soft dg sound, the rough edge of the t softened by the long oo).

This can be observed in other languages, as well. In German, scheisse hisses with its dual fricatives, and the Italian cazzo starts off with a hard k and grates with its harsh ts sound in the middle.

So, from a purely linguistic point of view, words that express someone's pent-up anger with short, sharp bursts of sound make good candidates for swear words.