George Costanza's ATM password in the Seinfeld episode "The Secret Code" is Bosco.
George refuses to tell his fiancée, Susan, the code because he treats it as a deeply guarded secret. Later, circumstances force him to reveal it while helping J. Peterman's mother, who is bedridden and near death. She repeats "Bosco" as her final word, leaving Peterman mystified about who or what Bosco is.
The episode is the seventh episode of Seinfeld's seventh season and first aired on November 9, 1995. Its central joke depends on George's strange reluctance to share a simple code and on the word sounding like a name or mysterious reference rather than an ATM entry.
A common mix-up is confusing Bosco with Costanza, George's surname, or with another character's supposed password. In the episode, Bosco is explicitly George's ATM code; the gag does not establish that it is a conventional word-based banking password outside the show's story.