A shamewank can be defined by the distraught feeling you get after masturbation, knowing that your belly is covered in cum having thought about the most disgustings thing ever.
Examples of these are:
Your best friends girlfriend
Your best friend (Same sex)
The 80 year old woman down the road
I caught young billie havin' a damn shamewank last night. Motherfucker had a picture of your sweet grandma in his left hand, while he beat his man meat and spunked on her glass covered pretty lips. Motherfucker!