Jack was the curmudgeonly farmhand at Sonny's Home for Boys.
Background
Jack was employed as a farmhand by the original owners of the farm that Sonny now owns, Howard and Doreen Wasserlauf. Howard believed that Jack and Doreen were having an affair. Howard got drunk one night and tried to kill Doreen and Jack with a meat cleaver. He killed his wife, but Jack escaped.
Season 9
Jack died in a freak accident involving a long-dead tractor that mysteriously roared back to life. Sonny calls Dean on the "bat phone" for help.