A good expletive should have harsh syllables and contain references to either obscenity or something gross. Since both the latter are not desired in this context, what you want is something that sounds like it /ought/ to be dirty, while not specifically meaning anything. :) My Texas relatives have been known to use "Mo-ther pussbucket..." but that's a bit too gross for me... "Festering fracklin..." "Oh, retch..." "Reeking drek!" Etc.