Originally a jewish saying which was commandeered by the famous television series "Star Trek" when jewish actor Leonard Nimoy used it as the Vulcan salute. It's now used as way for us nerds and sci-fi lovers to say goodbye in a different, more permanent way.
Friend moving away to another country - "i'm gonna miss you, man". Nerd - "Live long and prosper"
He doesn'ttell you how to accomplish that, though.
Leonard Nimoy was famous for saying, "Livelong and prosper." He himself seems to have accomplished that thanks to his good looks and pleasant personality, but I wish he would have explained to all of us about how to do it ourselves.
Short-hand for the word floccinaucinihilipilification, meaning the estimation of something as valueless (encountered mainly as an example of one of the longest words in the English language).