A random exclamation shouted to a person or group of people in order to express nothing in particular, other than one's presence. 1
Can also be used as a means of starting or continuing a dwindling conversation. 2
Derives from the common name for the Euphorbia genus of plants: the plant is an illustration of randomness and lack of neccessity, just like the word itself
Mike: "Hey man!"
E.G. 2 Harry (to Tim on, for example, Facebook Chat at the very start of a conversation): "Spuuuuurge!!"
Tim: "Spoouurrge!!!! How's it goin man?"