1) A brother-word to damnit, but used when you wish to have a shakespearean accent.
2) Most commonly used when yelling about something/someone that/who has made you upset enough to begin sounding like a 18th century play writer.
3) Also can be used if in highschool in attempt to get away with cussing in front of a teacher. You will later regret trying such adolescent things, but it is possible to laugh at later in life. Once out of highschool, it is safe to use without the feeling of stupidity which follows.
Damnith, I can't believe he screwed my mom.