Paint the town red
very enjoy nightlife with lots of drinking

Pandora box
Going to make mistake that cause huge danger

Panic stations
Very exciting situation

Paper tiger
Look powerful but has no actual power

Parrot fashion
Learn like a parrot. Do not understand but just remember

Pay on the nail
Pay cash right away

Pass the hat
Ask someone for the money

Pay the piper
Receive the consequence of what you have done

Pay through the nose
Pay in a very high price

Pep talk
Cheer up, encourage

Piece of cake
Very easy

Pie eyed
Very drunk

Pie in the sky
A promise that listeners expect that it will actually happen.

Pigs might fly
Believe something is impossible

Play hooky
Not going to school with no good reasons

Play second fiddle to someone
Deal with someone who are more important than you

Poker face
A person who doesn't show any emotion on him/her face

Pull strings
Try so hard to influence someone

Pull someone's leg 
Tease someone

Put on airs
Pretend to do something to impress someone

Put someone up at
Give someone the place to live

Put up with
Tolerance with

Put wool over someone's eyes
To deceive someone from knowing the real truth