When a man suddenly gains enough cocky bravery to face a dilemma or challenge which he was previously afraid to do.
Such a feeling simulates the results of one's testicles expanding due to a tumor, which explains the combination of the words "Man" and "Cancer". With his newfound "big balls", a man will stop being a whiney bitch and achieve his goals.
One experiencing Testicular Mancer will gain a short term badass attitude, shortly followed by sweating, crying, and proclaiming "Holy shit, did I just do that?!"
Paul: "Hey, did you hear? Randy just asked out Judy on a date!"
John: "Man, that guy has come down with serious case of Testicular Mancer..."