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!

Lying through your teeth

'Lying through your teeth' means you are completely and totally lying, probably in someone's face. It's stronger than just saying that someone is 'lying.'

Example:
-I stayed home last night, honest.
-You're lying through your teeth! I saw you at the bar last night!