Full of it
Definition 1 of 2
Pro Tip 1/3Emphasis on Dishonesty
Use 'full of it' to stress that someone's words are far from the truth. 
After hearing another excuse, I knew he was full of it.
Pro Tip 2/3Sarcasm and Humor
It can be used sarcastically or humorously between friends. 
You caught a huge fish with just a string? You're full of it!
Pro Tip 3/3Informal Context
Appropriate in casual conversation, not in formal situations. 
In a job interview, avoid saying someone is 'full of it.'