- "The front door opened and Biff walked out. / 'Biff!' the elderly woman's voice called from inside the Tannen house. 'Where are you going, Biff?' / Biff started to walk quickly away from the house. / 'To get my car, Grandma!' he called over his shoulder. / 'But when are you coming back, precious?' his grandma whined. 'My feet hurt, and I want you to rub my toes some more. And put polish on them!' / Biff waved violently back at the house. / Marty called back to Doc on the walkie-talkie. 'Never mind, Doc. Biff lives here all right.'"
- —From Back to the Future Part II by Craig Shaw Gardner (quote, pages 141 and 142)
Gertrude Tannen was the mother of Irving Thomas Tannen, and she was likely the mother of Frank Tannen as well. By the 1930s, Irving Tannen had become a gangster during Prohibition and began to refer to himself as Kid Tannen. He ran a speakeasy in Hill Valley, first simply known as the Hill Valley speakeasy.
After Kid Tannen's first speakeasy was burned down by an arsonist in 1931, he created a more elaborate speakeasy under the Sisters of Mercy Soup Kitchen named El Kid. This second speakeasy didn't last long, as he was arrested that year by Officer Danny Parker for tax evasion, using documentation provided by Kid's girlfriend Trixie Trotter after she learned about accounting from Arthur McFly.
Kid Tannen escaped from jail in 1936, and had a fling with a woman named Myra Benson, who gave birth to Biff Tannen. He wanted her to have an abortion, but she wouldn't do it as her mother would kill her, so he escaped from prison again in December. He then married Myra, using the name Thomas Tannen, simply to make it appear as if Biff wasn't born out of wedlock.
The two parted ways immediately after the wedding, and neither parent wanted Biff. Because of this, Kid gave custody of Biff to his mother, Gertrude Tannen. Before he left his son, Kid gave Gertrude a forged birth certificate that stated that Biff was born in 1937. From that point on, Gertrude Tannen lived with Biff at their house in Hill Valley.
Gertrude was known to be an extremely unpleasant woman, and did not seem to encourage visitors. This was supported by the fact that a large handwritten sign at the gate read NO TRESPASSING / VIOLATORS WILL BE PROSECUTED / THIS MEANS YOU!!, while a handwritten sign reading KEEP OFF THE GRASS! was also stuck into the front lawn.
In 1955, in an alternate timeline, Biff received a sports almanac detailing fifty years of sports statistics from his older self. Since he was only seventeen, he couldn't bet, so he gave money to his grandmother to bet on the Chicago Bears. She didn't bet on the Bears, and lost, making Biff twenty dollars poorer. Biff told her to make another bet, on the Washington Redskins. However, she only gave Biff back his twenty dollars, and kept the rest for himself. When Gertrude claimed Biff was as greedy as his father, Biff stated that he learned that from her.
Biff later was approached by a shady man named Uncle Lou, who saw Biff using the almanac to tell his grandmother which teams to pick. He killed Gertrude, and put her in the back of his car to bury her. He planned to steal the almanac, kill Biff, and bury him next to her. However, when he went to hit Biff with a tire iron, Biff caught it and killed him instead. Biff justified the murder in his mind based on the fact that Lou had killed his grandmother. He buried Lou, and took all of the money that was in his briefcase. With the money, he covered up the murder of Gertrude by paying to have a death certificate forged that stated she died of natural causes.
Later at school, Lorraine Baines offered her condolences for the loss of his grandmother, and Biff tried to proposition her, before clumsily trying to apologize for his actions at the Enchantment Under the Sea dance.
Biff then tried to use the death of his grandmother as an excuse to principal Stanford S. Strickland for his repeated absences. When Strickland told him he would be repeating his senior year for the third time, Biff tried to bribe him with some of Lou's money. Strickland then expelled him from school, and Biff stated that he intended to drop out.
Behind the scenes
- Gertrude Tannen was never shown on-screen, but her voice was provided by Thomas F. Wilson, notably where she is yelling at her grandson Biff (also played by Wilson).
- Tom Wilson stated in an interview that the incessant nagging from Biff's grandmother may be the primary reason Biff grew up to be so mean. She could be heard yelling to Biff the night of the Enchantment Under the Sea dance.