When someone is full of
piss and
vinegar, they are in a very bad,
bile-rising-in-
throat mood.
For example, whenever I'm stomping about the house in a generally angsty teenage mood, my mom will warn my sister, "Watch out! The girl is full of piss and vinegar today!"
I've heard that 'full of gar' can be used as an abbreviation as well, but I think it's much more fun to just say piss and vinegar.