Jack Smith is Stan Smith's father.
According to "Con Heir", Stan believed his father to be a top secret agent in "The Scarlet Alliance" while in reality, he was a jewel thief. After the man Stan paid to pretend to be his father died, his real father dropped in on the family, literally, jumping from a helicopter while the family was at the funeral of "Grandpa" Smith.
Roger developed a "boy crush" on Jack but the other family members were wary of him. When Jack returns to Stan's life, taking him out to a gun range. Jack talks Stan into joining him for a mission to, supposedly, stop terrorist activity. When they arrive at the "National Gallery of Art" Stan realizes that they are actually on a jewel heist and is betrayed by his father.
It was said that Jack Smith had spent 20-40 years in prison and had been in prison in "Jack's Back", besides being in prison since Stan's childhood it had been believed he was in hiding.
However Jack also had a softer side and when he realized that Steve saw him as a greater role model than Stan he turned himself in to the police, telling Steve in "Jack's Back" that he'd never been a good father to Stan but he wanted Stan to be a good father to Steve. However when Stan tried to hug him, he kicked him and then explained that he had to put on an air of dominance or the people in prison would "eat him alive". Jack also ruffled Steve's hair affectionately, showing that he did feel some affection for family members.
Francine recounts the story of Stan's father leaving him in a flashback in "Oedipal Panties". Stan discusses his parents break up with Steve while posing as a bully in "Bully for Steve". He is also shown in a flashback leaving in "Stan's Best Friend".
To keep "Takin' Care of Business" from returning Stan's memory in "The Boring Identity". Francine makes up a story that his father was raped to that song to keep him from listening to it.
When Steve misbehaves in "Minstrel Krampus", his grandfather Jack tells the story of how, as a boy growing up in Lower Berchtesgaden in Southern Bavaria, he captured the legendary Krampus, Santa Claus' sidekick that punishes naughty children. Tired of Steve's brattiness, Stan allows the Krampus to go free but he kidnaps Steve and demands Jack be brought to him. But when Stan uses his CIA connections to free Jack, he instead flees, leaving Stan and Roger to try to make a deal with Santa who still bears a grudge after the events in "For Whom the Sleigh Bell Tolls" but is willing to go along in order to kill the Krampus.
Meanwhile, Steve discover that the Krampus only punishes children because he loves them and tries to set them straight and bonds with him, just as Stan and Santa arrive and Santa kills the Krampus before turning the gun on Stan. Jack, who has had a change of heart, takes the bullet meant for Stan. As Santa flees, Jack's and the Krampus' blood intermingle, transforming Jack into the new Krampus.
When Steve turns the tables on Stan and traps him in the CIA hologram deck in "I Ain’t No Holodeck Boy", he has Stan relive some of his childhood including times before his father left him.
"Con Heir" "Dungeons and Wagons" (mentioned) "The Best Christmas Story Never" (trip to the past) "I Can't Stan You" (mentioned) "Joint Custody" (mentioned & heard) "Oedipal Panties" (flashback) "Man in the Moonbounce" (mentioned) "Bully for Steve" (mentioned) "There Will Be Bad Blood" (mentioned) "Jack's Back" "Stan's Best Friend" "American Stepdad" (mentioned) "Blood Crieth Unto Heaven" "The Boring Identity" "Minstrel Krampus" "I Ain’t No Holodeck Boy"