A carbo-dook is a contraction of 'carbonated dookie'. This can be seen when one takes a massive dump and for some reason said dookie has small bubbles coming off of it in the water.
Scientists have pondered as to why this happens for centuries. Some scientists suggest it is because of an excess of carbonated beverages in one's diet, whereas other scientists say those scientists are totally gay.
Bob:Yeah so Bill's mom was being a total shig to me the other day?
Tom:junx! What did you do, dude?
Bob:Well I left her a carbo-dook in her washing machine and then I broke!