Knowing my players, especially when playing Grogs they'll be using colorful and creative language.
I, however, would at least like it to be in-period so as not to make my Saga sound like a bad/classic 90's R-rated action movie ala Die Hard or Terminator.
So, what might a medieval person use as an epithet, exclamation, or colorful phrase? From crude to common foul language.
If region means anything, we're looking at Moorish Spain, the Rhine, or perhaps the Levant.
So, taking that as you will, proceed to turn my cheeks red.
I've sort like the fact that our Ars Magica game reads like a cross between a Whedon and Tarantino film. It's not in-period, but some of the stuff our players/characters say is hilarious! Sometimes this is the main reason I play.
Hm, true enough. I just want a nod towards immersion. Latin does that a bit, I guess... but peppering in a few real words or phrases might bring home the fact that 'you're not in Kansas any more'.
"Swive". The exact medieval (middle-English) version of the F word.
Swive yourself.
Get swived.
Swive off.
Swive it all.
We're all swived.
You get the idea.
"Bloody" is another one- altho' mild by today's standards, it refers to "Bloody Christ", and so is sacriligious as well as extremely strong. Similar to...
"God's Wounds" (mentioned above), which is sometimes abbreviated into "Swounds" (also where the exclamation "Zounds" comes from, tho' pronunciation has changed over time with that one.)
...anything referring to Our Lord's body seems to have been considered a wicked cool way to blaspheme/curse, perhaps causing Medieval knights to give each other high fives.
Like "God's hooks" (referring to the nails used to crucify Jesus, laters gadzooks), and "God's teeth," which was my companion character's favorite oath.
Generally blaspheme, ill-wishing, and accusations of being animal-like are good ones.
Although the tendency isn't as much as it once was, I heard from a former German teacher of mine (who lived in and still travels Germany) that being called 'bull' or 'goose' or assorted things is considered insulting (Back then, it would have been all the rage, although now it's been primarily subsumed by American swearwords) As a teenager, she was somewhat surprised and mostly amused.
Also try drawing attention to uglinesses or social status. Insult something they're very protective of.