Contraction of damn it. Most often it is used when
someone is generally pissed off at everything, it's worse than just saying the words separately.
blink182's most
popular song.
The man almost got run over by a car, got into a fight with a trucker, broke up with his
girlfriend, got fired, got arrested, got drunk, crashed his car, locked himself out of his house, and tried to climb in the window but fell off and
broke his
leg. When it started to rain, the man said DAMMIT!