Arnold Shemp "Arnie" Thomas was a poker-playing buddy of Dan Conner and the ex-boyfriend of Nancy Bartlett (played by Sandra Bernhard) on Roseanne. Although they later married, they later divorced under less than amicable conditions. He later returns to ask her to remarry her in Season 5 after leaving her in Season 3. He was played by comedian Tom Arnold, Roseanne Barr's real-life husband at the time. He appears in 19 episodes between Seasons 2 and Season 5.
About Arnie and Nancy
Arnie is the overweight, sometimes hot-headed, jovial friend of Dan Conner. Originally, he was written as a relative stranger to both Roseanne and Jackie, but this was later retconned and it is revealed he wnt to high school at the same time with them so perhaps they initially just didn't remember him. He frequently cheats on the women with whom he dates, and personality-wise, he is generally very ill-mannered and uncouth in how he responds to others. However, Arnie always tries to be a good friend to Dan. He eventually borrows money from Dan to get Nancy breast implants, a revelation that frustrates Roseanne because she and Dan are often struggling. Although Arnie eventually marries Nancy and takes her with Roseanne and Dan on a trip to Las Vegas, he later leaves her, claiming to have been abducted by aliens. (This preposterous claim is later dramatized in the fourth-season finale end-credit tag, where he is seen conversing with aliens on a spaceship, revealing it possibly actually happened.)
Before Arnie and Nancy were engaged, he had a one-night stand with a Jackie while she was intoxicated. He is often seen wearing a yellow University of Iowa sweatshirt, which is possibly second hand as he has never mentioned having attended the university. (In reality, Tom Arnold had attended the University of Iowa in real life) He is last seen in Season 5, where he unsuccessfully tries to win Nancy back, even after finding out that she is a lesbian.
Tom Arnold appears in the ending credits of a later episode, but he is now playing Jackie Thomas (His role on The Jackie Thomas Show.), and none of the characters seem to recognize him.