to find something fantastic or is totally you (William), but only a Will or William can say this to himself. otherwise someone not with the first name William can say as a compliment to the William. Not usually used in sarcasm.
"OMG this is so willtastic!"
"This shirt is totally willtastic."
"That date was so willtastic last night!"
Person 1: “Hahaha that’s funny buy you don’t t have the balls to do that”
Person 2 : “ actually yes I do I’m willytastic ” *shows them penis and slaps them*