Full of shit

Someone who is 'full of shit,' is someone who is completely lying. Since 'shit' is a bad word, obviously this should only be said around people with whom you have a close relationship.

Example:
-I can't believe she actually said that! She's so full of shit!
-Totally!

P.O.S.

'P.O.S.' refers to something that is a 'piece of shit.' It is usually used to describe an old car, but can be used for other things as well. It is pronounced letter by letter, not as one word.

Example:
-Dude, you really need to get a new car!
-I know! I'm so sick of this P.O.S.!

Bat-shit crazy

'Bat-shit crazy' is used to describe someone who you think is just plain crazy and beyond any hope of getting better. In reality, it's not usually used to describe a person who is truly mentally ill, but rather a person who has such outlandish, uninformed and offensive beliefs that you're sure they must be crazy.

Example:
-That presidential candidate is bat-shit crazy!
-I know! If he wins, I'm moving to Canada!

Shit-faced

'Shit-faced' means really drunk.

Example:
-Last night we got so shit-faced!