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!"