I don't see the name explained in that document. It does mention 'C-Scheme' and 'Z80 Scheme' so maybe Chez is a sort of interpolation of those?

In French chez means "at the home of (used in imitation of French, often humorously)." This always rings this bell in my mind when I hear this word.

An example: Je suis allé chez Steele et Sussman roughly translates to: I went to the Steele's and the Sussman's homes

^ this paper is mind-expanding to see the huge evolution and depth of effort around performance optimization over years for this system