Medium-strength Japanese curse word roughly translating to "shit". Not used to describe yourself or others, but solely as an exclamation. Example:

Dragoon: Shimatta! Mahou chibi ohimesama wa kowai!

English translation: Shit! The magical little princess is scary!

This one is offensive, but not wildly so. Still, don't use it except with friends or people who are probably just as rude as you.

Thanks for the correction, Starrynight.