1. [from the soc.motss newsgroup on Usenet, after some obnoxiously gooey cartoon characters] A newsgroup regular with a habitual style that is irreverent, silly, and cute. Like many other hackish terms for people, this one may be praise or insult depending on who uses it. In general, being referred to as a smurf is probably not going to make your day unless you've previously adopted the label yourself in a spirit of irony. Compare old fart.
2. [techspeak] A ping packet with a forged source address sent to some other network's broadcast address. All the machines on the destination network will send a ping response to the forged source address (the victim). This both overloads the victim's network and hides the location of the attacker.