Arnold Shemp "Arnie" Thomas was a poker playing buddy of Dan Conner and the boyfriend and then estranged husband of Nancy Bartlett (played by Sandra Bernhard) on Roseanne who returns to her to remarry her in Season 5 after leaving her in Season 3. The part of Arnie is played by Tom Arnold, Roseanne Barr's real-life husband at the time. Arnie appears in 19 episodes from Seasons 2 to 5.
About Arnie and Nancy
Arnie is the overweight, sometimes hot-headed, jovial friend of Dan. Originally, he was written as a relative stranger to both Roseanne and Jackie, although this was later retconned, and he was subsequently written as having gone to high school with them. He frequently cheats on the women he dates and is very ill-mannered. However, Arnie always tries to be a good friend to Dan. He marries Nancy but leaves her, claiming to be abducted by aliens (later played upon in the fourth-season finale's end-credit sketch, where he is seen conversing with aliens on a spaceship).
Before he and Nancy are engaged, he has a one-night stand with a drunk Jackie. Arnie is often seen wearing a yellow University of Iowa sweatshirt (Tom Arnold 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 is now playing Jackie Thomas (in his role on The Jackie Thomas Show), and none of the characters seem to recognize him.