Water. A tongue-in-cheek joke derived from the creation story in the Bible. Adam was the first man in the Garden of Eden; he lived a pure lifestyle in communion with God. Before the fall, he was unclothed an eating just fruits and vegetables in the garden, therefore the only thing to drink was water.
Xander is an extremely rich and not posh person who enjoys playing copious amounts of halo in his free time. Xander also enjoys complaining about his parents, saying he's not posh and eating his corn raw.