What does Boxing Day have to do with boxing?
A telephone system cracker. A phacker may attempt to gain unauthorised access to a phone system in order to make free or untraceable calls or he may disrupt, alter or illegally tap phone systems via computer.
The disruptions may include causing a phone line to be engaged so no calls go in or out, redirecting outgoing or incoming calls, as well as listening to actual calls made.
Phackers are frequently confidence tricksters or phone freaks (nuisance callers who can only relate to other people by phone). Phackers are sometimes employed by illegal enterprises to conduct business using untraceable calls, or to disrupt, or follow legal authorities' investigations. Phackers interventions may be lethal to the person being phacked.
A phacker may be a phone company employee, or usually, ex-employee who specialises in illegal phone system disruption, alteration or tapping via physically altering installations. A phacker is generally considered to be a socially and intellectually retarded cracker.
See Captain Crunch.