Amanda: How much did you pay for that painting?
Kevin: It's an antique. I paid 800 dollars!
Amanda: You are such a mug! I saw that picture in Tesco last week for 5 dollars!
Mug - English slang for a victim - somebody that is easily tricked.
The word mug has other slang meanings, depending on the context of the sentence.
The verb mug means to threaten violence to a person with the intent to steal from them.