A person who sucks at life. They are incapable of performing basic and simple daily tasks (with the exception of masturbation).
You have your left turn signal on and are clearly trying to merge into the left lane; instead of slowing down to allow you to change lanes, the Jackwacker behind you speeds up to block your access. This guy is a complete Jackwacker; he sucks at life.
A homosexual activity, which involves an individual "Jacking off" a minimum of 10 other gentlemen in a single night/party/event. Only once all 10 men have ejaculated can the act be called a Jackzacker.
Brad: Last night at the club i Jacked off 15 blokes
Shane: thats a great Jackzacker-ing
Brad: yep once I hit 12 my right wrist got sore so I had to use left
Shane: is that a record?
Brad: yep my jackzacker record was 13 not its 15!