Dame Judith Terwilliger (née Underdunk) is the wife of Robert Terwilliger Sr. and the mother of Sideshow Bob and Cecil Terwilliger. According to Lisa, she is a retired Shakespearean actress. Her sons get their extremely curly hair from her. Though she looks like an elderly female version of Sideshow Bob, she and Cecil share a similar nose structure.
The title "Dame" means that she has been knighted, likely for services to theater.
On that note, she is one of only three Terwilligers to not be voiced by one of the actors from Frasier, the others being Gino and Francesca.
Prior to her debut appearance, she was mentioned in a special promo for Brother from Another Series, where Sideshow Bob angrily reminds Cecil that their mother had high hopes of them being prominent psychologists, an obvious reference to their respective voice actors portraying the Crane brothers in Frasier, who were both psychiatrists.