The act of jerking off (typically a man), especially to the point of orgasm.
Originates from the term "jacking off" as well as the similarity of the "cracking" sound from performing it and the opening of usually alcoholic beverage bottles.
It is also a popular perversion of "cracker jack", the brand of caramel corn.
Roommate: "You two were so loud last night! I heard you cracking jack so loud I couldn't get to sleep!"
"Yo, she was cracking jack so hard i almost passed out!"