According to Jim: The Bad Word

Cheryl leaves Jim in charge of their kids and some other kids who were there to play with the girls during a Bears game. The next day, one of the kids mom calls Cheryl saying her daughter dame home repeating a swearing she heard from Jim. Cheryl apologizes for him and Jim is insulted that she always automatically takes the other person's side. Cheryl apologizes, and not too later Grace repeats the exact same word. Jim decides to teach her not to repeat it by making her say it non stop until she can't say it again. It works. 5 days later, at Ruby's ballet presentation, she slips, falls and says the word, unmasking Jim's guilt.