I need a fantasy word for "Smarmy" and "Jackass"
 

Pretty much what the title says. A few extra fantasy appropriate words for "bitch" and "womanizer" couldn't hurt either.

I'm seeing if I can have the main character and a romantic interest start off hating each other, but I don't want to overuse the word 'idiot' before I get halfway through.

Deryk- Or something pronounced similar. It can sound condescending plus you'll get a warm fuzzy feeling knowing that anyone named Derrick will now feel insulted when someone calls out their name. Okay not really, but you can pretend you're insulting people named Derrick.

do you have any idea of how hard it is to find Medieval Slang online? damn! thought it would be easier...

I think the best way for you to go now, would be to look for Irish or British slang, and go as old as you can... it might give your characters an "accent", but it fits the motif.

Plus, you can have her call him a "Spotted Dick-head", and be in context

I dunno, you might end up veering too far into Paolini-esque thesaurus rape. Is there running usage of similar fantasy words throughout the book, or is it just for this section?

Although, I guess a good way to end the argument would be for one side to throw out a ridiculous word and shut the other side up while they try to puzzle out what they were just called.

Well, what he did was over complicate things where it wasn't necessary. Mirai's looking for synonyms of insult words that fit the context, which itself isn't really all that complicated in the first place. Using a thesaurus helps, but you gotta be careful on which words you decide to choose.

As for my contribution, maybe using wench or something.

Bitch-harlot and womanizer-knave seem pretty easy.

That said, it depends on the characters. I recommend a Cockney rhyming slang, myself!

I've got nothing for fantasy, but is a Shakespearean insult good enough?

Shakespearean is way too far out for what I'm looking for.
Yup. Paolini kept on seeking different words each time he needed them; I just need a handful they can use over and over to mock each other.

Knave I think could work, but harlot kind of transcends that into 'whore', so I'm a bit hesitant there.

look up curse words in other languages Butcher the spelling
baka=fool=Bac'a or something along those lines

I'm not too sure what you mean by "fantasy word", but couldn't you say "ass" instead of jackass and a very derogatory "woman!" instead of bitch? Although I'm not sure why you can't use "bitch" as well.