vent (your) spleen

meaning: to say all the things that make you angry about something; to release all your anger

1. It was usual on Friday afternoon for the boss to vent his spleen at one of the interns. That is why he had them in the office.

2. “I’m sorry, I didn’t mean to vent my spleen like that. I guess I’ve just got a few things on my mind.”
vicious circle

meaning: actions that make each other worse; a cycle of actions that keep making things worse

1. Once Dave started to drink, it was a vicious circle. At night he would get drunk to forget his misery, and then in the morning wake and feel more disappointed in himself. That would make him want to drink again.

2. The success of the economy has actually started something of a vicious circle in terms of traffic and pollution: as more people have become rich, so they have bought more cars. These cars needed more roads, and so nature was destroyed to build them. More roads, however, has only encouraged more people to drive.
virgin territory

meaning: an area that hasn’t been discovered or explored before

1. “I’ve exercised before, but body-building is virgin territory for me.”

meaning: a complete sudden change to the opposite opinion

1. The Prime Minister did a volte-face on immigration once he saw it would win him more votes.