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5.
"Half Jack" refers to a person who is gender confused, and possibly considering sex change surgery. A person who either feels that they are of the wrong gender, or that they are half male and half female.
Mary was really into Tom at the party last night, but she wasnt aware that he's "Half Jack" and had no interest in her.
by Scott the Heirophant February 09, 2008
 
1.
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."

or

"Damn! He only finished half the fence! What a halfjack!"
by Thereyougo. April 03, 2010
 
2.
You get so uninterested in masturbating, that you stop before you cum.
Peter: Dude! I was so baked last night, I was whacking off and got bored and stopped.

Paul: Just a half-jack then?
by Goldfish408 March 03, 2010
 
3.
The process of exciting male genitalia until it is partially erect in order to help the refractory period after getting the first "easy" orgasm out of the way.
Joe half-jacked so that later when he has sex with Jennifer Whoreton he would be able to reduce the time between sexual excursions.
by LanguidLover July 18, 2011
 
4.
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
by Grem Lochington January 25, 2008