A nice way of saying:
"I understand/know a lot of complicated bullshit that you couldn't grasp even if it were your dick after 8 hours of hot lesbian porn."
It was once believed PhD was actually French for "Fuck off idiot," but this was actually proved wrong by brain-box PhD holders after years of research.
Some guy: Ugh, I can't solve this 3rd order partial differential equation...
Professor "blah, blah", PhD : I can, thus my mental-dick is bigger than yours. Checkmate.