It refers to a woman that has the inability to commit to crossfit classes and therefore decides to burn calories by means of masterbation or phoning a "friend". Characteristics of a Master Wanker are that she mysteriously disappears during lunch (a nooner) or early afternoon (afternoon delight) and reappears with a sneaky smile on their face.
After weeks and weeks of fooling herself the Master Wanker looks in the mirror and realizes that she is hot enough and decides to go back to being a normal wanker.
by The "Master" Baiter August 19, 2011