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Kind of like fucktard, fuckhead, stupid fucking idiot, only better since without the "retard" of connotation inherent in "fucktard" and not as lame as douchbag/douche, etc.
"How about those fuckjacks (n.) who try to force their entire torso into the subway doors before they close?"

Also, as a verb:

"to describe the action of jacking the subway door open and screwing everybody else. "I totally fuckjacked my way onto the train this morning, and then some dude in a cape took a boot to my sternum."
by pikadar May 18, 2010
a word used to denote the use of alcohol when trying to get laid. a conventional jack raises cars so that tires can be changed. a fuckjack (a drink) assists in the spreading of a girls thighs.
i can't score with this chick, so i may have to incorperate the use of a few fuckjacks to loosen her up.
by ken power July 29, 2008
A word to define someone for whom the word "jackfuck" does not do his/her true description justice
"Neil, you...you...fuckjack."
by Benji December 11, 2004