What is the meaning of Lines 91-92: "She wet her fingers lightly in the dish / Of sauce, for courtesy was her first wish." Why does she wet her fingers with sauce?
The quotation (with an important correction) to which you refer is part of The General Prologue to The Canterbury Tales (ca. 1387-1400) and describes the Prioress, Madame Eglentyne, whose name immediately marks her as a refined and worldly woman:
And French she spoke fairly and elegantly,/As taught at the school of Stratford-at-Bowe,/For the French of Paris was unknown to her. (ll.124-126) (my translation)
The character of the Prioresse is part of Chaucer's subtle satire upon various "estates" of late-medieval England, in this case, against the Church. The fact that he gives her the name Madame Eglentyne, a very elegant and worldly name, puts us on notice that she will be more than a typical prioress. Her knowledge of French--even though it is provincial, not Parisian, French--indicates that Madame Eglentyne identifies herself with the aristocratic classes, descendants of the Normans who conquered England in 1066. A prioress in 1387 would need to know Latin and English, not French.
Among her aristocratic attributes are her table manners:
She let no morsel fall from her lips,/Nor did she wet her fingers deeply in the sauce bowl. (ll. 128-129) (my translation)
You will notice the addition of the word Nor in the line. In the original Middle English, the line reads, "Ne wette hir fyngres in hir sauce depe." Consistent with her elegance, her knowledge of an aristocratic language (French), the Prioress's table manners are also faultless: unlike less elegant pilgrims, Madame Eglentyne does not dip her fingers deeply in her sauce bowl so that her fingers get wet. In Chaucer's time, most meals eaten by the middle class (which makes up most of the pilgrim group) were eaten with the hands, not with a knife and fork, so a sign of good breeding and elegance is the ability to eat without getting the fingers wet with grease.
The line in question, then, contributes to the depiction of the Prioress as a woman much closer to the world of aristocrats than to the isolated and strict religious life of a prioress.