1. To get any current work or job half done.
2. Someone who only finishes half their objective.
"This car has been half built, looks like we got a halfjack on our hands."
"Damn! He only finished half the fence! What a halfjack!"
1. One who never goes more then halfway on anything
2. A boyfriend never planning a date and leaving all the work to his girlfriend and more commonly referred to as a jackass
3. One who has winterfresh on a daily basis and swallows
EX 1. Paul never planned anything with his girlfriend so he is a halfjack / jackass
EX 2. Paul likes to swallow winterfresh