You call someone a Willy D. when all other insults have been exhausted. This one is very vile and should only be used as a last resort. To use this term in a derogutory sense is to combine the words poser, misfit, wannabe, friggin annoying bitch, ego-tripper, mr. arrogant, fugly, stalkerish, conceited, perverted, obsessive, and all and all creepy. So if you know someone who fits all the above, and just can't take a hint, don't hesitate to refer to them as a Willy D.
Omg that guy is so perverted! Janet, I'm sorry he's so obsessed with you... I swear he's gonna kill someone someday... he's such a weird, wannabe, freak... what a Willy D.