Per
Ossobuco's request...
dysphemism: an offensive or disparaging expression that is substituted for an inoffensive one
It's the opposite of a euphemism. You take a term and replace it with something that sounds much worse. It's like referring to an asylum as a "loony bin" or to cigarettes as "cancer sticks."
Got any? Here are some of mine:
Yani = "sonic wallpaper"
bacon = "sticks o' swine"
a Ford Pinto = "Gas Tank On Wheels"