Postby thegooseking » February 1st, 2014 11:42 pm
gnossienne3さん、
んだもん is short for のだもの.
のだ on its own means "it is the case that..." or "the fact is that..." and もの means 'thing', so のだもの literally means "the thing is..." and, just like in English, indicates that the sentence is a reason or excuse for something.
小狼