Best definition
You don’t know Jack…
Abbreviation for a popular computer game available in the 1990s and 2000s both online and as a boxed game. It contains pop culture questions, often phrased in a manner laden with innuendo.
Derives from the slang phrase “you don’t know jack shit”.
A bunch of friends are coming round for a couple of beers and a pizza and we’re gonna play YDKJ. You up for it?