One who intentionally (a rude racer) or unintentionally (an inexperienced driver) crashes ("hacks") into other vehicles on the track. A hacker can be easily spotted and sometimes disqualified at the race directors discretion. If you're having radio problems, pull yourself off the track so others don't leave disappointed because your car kept hitting them. If your just not experienced enough, get some practice before entering a race.
Hacker is also a company, Hacker Brushless USA, who manufacture brushless motors and speed controls.