jackSpecifically, I was looking for a definition to support the expression, "You don't know jack."
Webster's New World had a lot of definitions, including terms for playing cards, car lifts, electrical sockets, applejacks, male laborers, and even slang terms for money and saying, "Listen here, Jack." But not the definition I was looking for. Ditto that for Merriam-Webster online.
American Heritage, however, did have it. Its very last definition for the noun — definition No. 15 — is:
Slang. A small or worthless amount: You don't know jack about that.
Funny. I sometimes worry that dictionaries (especially you know who) are a little too hasty in adding new words. But on "jack" it feels to me that they're lagging behind. Of course, that's purely prejudicial and due to the fact that I use this form of "jack" so often. (I'm sassy like that.)