WASHINGTON — One of the abiding mysteries at the Supreme Court is why Justice Clarence Thomas has failed to say a word in almost seven years of arguments. On Monday, when he finally broke his silence, the mystery was replaced by a riddle: Just what did Justice Thomas say?

The justices were considering the qualifications of a death penalty defense lawyer in Louisiana, and Justice Antonin Scalia noted that she had graduated from Yale Law School, which is Justice Thomas’s alma mater.

Justice Thomas leaned into his microphone, and in the midst of a great deal of cross talk among the justices, cracked a joke. Or so it seemed to people in the courtroom.

The court transcript confirms that Justice Thomas spoke, for the first time since Feb. 22, 2006. It attributes these words to him, after a follow-up comment from Justice Scalia concerning a male graduate of Harvard Law School:

“Well — he did not — .”

Although the transcription is incomplete, some people in the courtroom understood him to say, in a joshing tone, that a law degree from Yale could actually be proof of incompetence or ineffectiveness. Others thought that he might have been referring to Harvard. What follows in the transcript supports the view that Justice Thomas made an actual point.