Jack is a fishing bait salesman who is mentioned in the season 2 episode Jumpin' Crack Bass (It's a Gas, Gas, Gas).
He is mentioned to Hank by Ray Holiday who tells him that he "wrote the book" on homemade bait and that he sells bait out of his truck on the corner of 6th and Woodlawn. Ray also mentions that Jack grew up in the "hills" and never went to school. On the way to purchase the bait, Hank mistakes a drug dealer to be Jack. It is then revealed that the real Jack's truck was just little further down the road.