Well here are some ways you can do just that, and surprise the hell out of everyone else with your thundering creativity.
Method #1: Invoke The Divine
Aside from our favorite four-letter friends one of the most common swears you've probably heard is the name Jesus Christ. While it might not offend anyone but the truly religious (taking the savior's name in vain and all that), the idea of invoking divine powers is one of the oldest forms of swearing. Given that most fantasy RPGs have their own pantheon you've got all of the ammunition you need.
|Asmodeus worshipers get the best swear words.|
You can invoke the name of your character's god(s), or just the regional divinities where he or she grew up. You can also invoke aspects of the divine, referring to divine tools or areas of influence. An example is the word gadzooks, which we sort of associate with Saturday-morning-cartoons and Shakespeare plays. The term is a shortened word that means god's hooks. That's some serious-sounding-shit, and not to be invoked lightly. Especially if you happen to be a follower of a god of pain, agony, and torture like Zon-Kuthon.
This kind of logic extends to all gods and their accouterments. Followers of Cayden Cailean might use a phrase like busted cup to refer to bad luck coming their way (insinuating that the god's trusty tankard had been smashed, and the ale of victory spilled forever). Those who worship the nature god Gozreh might invoke the more damaging elements of nature with phrases like thunder and flood or beasts and blood. Even those who pray to Desna the goddess of dreams might say something like dark dreams or sleepless nights when something goes wrong.
Method #2: Stream-of-Nonsense Swearing
Popularized by characters like Yosemite Sam the stream of nonsense might sound like little more than gobbledy-gook at first, but it actually has a long and storied tradition. Phrases like by the double-barreled jumping jimminity are more likely to produce laughter than fright, but the formula is easy enough to apply if you want to alter the words for something a little more serious.
|By the nocked arrows of the 52 fletchers, what do you want?|
Whether these phrases refer to actual things and people, or they're just made up combinations of words is up to you as the player. The formula tends to start out with by the as most good swearing does, and then moves onto words that come in pairs. Alliteration is good if you can manage it, and it will make the phrases easier to remember.
The interesting part comes when you want to work something into your character's life that makes these phrases make sense. Do they swear by weapons because warriors hold those close (by the bastard blades of the seven sons), or do they instead invoke features of their geography (by the burning hills of Hartford), or of their ancestors (by the gray bearded chin of Harold the Mad). You can get pretty creative with these, but you should save them for special occasions when possible if you want them to have a bigger impact.
Method #3: Swearing By Body Parts (And Other Things)
This is the type of swearing we're all most familiar with, though we aren't really all that creative with it. After all a four-letter word for copulation is our big swear word, and the runner-up is another four-letter word for excrement.
We can do better than that.
Look at the proud British tradition of swearing. Bollocks might not seem like a very insulting word, but try grabbing your crotch while you sneer derisively at someone and see just how quickly it escalates. The same goes for the word bloody; it probably sounds quaint to American ears but if you change the phrase and add to it something like blood and ashes has a certain vehemence that's got a lovely ring to it. You can swear by anything that's important to your character or the culture he or she comes from. Virgin's teats, devil's hands, or even something like child o' mine might represent things that a culture values, or which it finds troublesome.
In short there's a whole slew of great ways to swear-without-swearing at your table. For some more great information on old-timey cussing and where our modern profanity comes from check out this article on the subject.