A masturbatory definition is a self-definition written by someone with a low self esteem or a huge ego. These definitions almost always exaggerate the attractiveness and/or sexual prowess of the subject. They are so named not because they involve a person actually jacking off to their definition (although they often do), but rather because, like masturbation itself, they serve no purpose other than satisfying the one person involved. Urban Dictionary is rife with these definitions, despite the "don't name your friends" policy, presumably because people who write masturbatory definitions have no friends.
Jack showed his peers the masturbatory definition he wrote for himself last night in hopes that they would find him a little more bearable.