A variation of "God damn it", usually used when one is in a state of bitterness, pain, or rage.
Bitter: "I used to go out with Gwen Stefani, but I broke up with her. Godammit."
Pain: "OW! GODAMMIT!"
Rage: "Godammit, I'll stab your face if you don't shut your talk hole!"
You: yes, i'm finally a millionaire.
Frank: oh hey dude, guess what i just printed a ton of money and gave it to everyone so there would no poor people left.
You: goddamnit frank