You’re thinking of dihydrogen monoxide. And yes, the stuff is lethal! It’ll kill ya if you get too much in your lungs!!
http://www.google.com/search?hl=en&q=dihydrogen+monoxide&aq=0&oq=dihyd
He’s making a play on Penn and Teller’s joke on hippies to sign a petition banning dihydro-oxide (or water) and how evil the most needed chemical can sound.