First time I've heard that expression in my life.
I'm going out on a limb and saying that Lydia Wheeler pulled the concept out of her rear end.
“First time I’ve heard that expression in my life.”
https://en.wikipedia.org/wiki/Solicitor_General_of_the_United_States
The Solicitor General, who has offices in the Supreme Court Building as well as the Department of Justice Headquarters, has been called the “tenth justice”[3] as a result of the close relationship between the justices and the Solicitor General (and their respective staffs of clerks and deputies). As the most frequent advocate before the Court, the Office of the Solicitor General generally argues dozens of times each term. As a result, the Solicitor General tends to remain particularly comfortable during oral arguments that other advocates would find intimidating.[citation needed] Furthermore, when the office of the Solicitor General endorses a petition for certiorari, review is frequently granted, which is remarkable given that only 75–125 of the over 7,500 petitions submitted each term are granted review by the Court.[4]