As a lover of language, I'm fascinated with the origins of the English language. In order to add a touch of olde worlde to my books, I like to add the odd medieval phrase. Admittedly, these phrases are not technically correct for the time periods I write in but it's a tool most of us historical authors use. While I don't go in for thee's and thou's, I do enjoy using mayhap, 'tis and 'twas's! As I told one person recently, when writing medieval dialogue, think to yourself 'How would Yoda say it?'. "I know not..." being an example.
Now I've yet to use any of these insults in my books. I can't quite imagine one of my heroines calling the villain a "goaty milk-livered maggot pie" but they're still fun to read and far more inventive than the insults of the modern day.
Here are a few of my favourite concoctions:
Roguish onion-eyed hugger mugger
Loggerheaded swag-bellied horn beast
Vain toad-spotted whey-face
I could go on for days but you get the idea. It's incredibly tempting to use them sometimes but I fear I would be carted away in a straight jacket so I've avoided it so far! Maybe one of these days I shall find a way to sneak one into one of my stories...