The title is a reference to a scene from the book between the two inmates, Molina and Valentin.
Molina tells Valentin about a a film he once saw, whether true or not, about a woman whose husband had been cheating on her and she had become afraid of instantly turning into a panther and devouring him if he kissed by her.
Later on, Valentin asks for a kiss from Molina before his parole release. The information from the FAQ reagarding this is below.
The title comes from a part in the book where Molina asks Valentin for
a kiss before he is paroled. Valentin (Raul Julia) asks Luis Molina
(William Hurt) whether he's afraid that he'll turn into a panther
woman (as in the first film Molina tells Valentin, where the panther
woman kills when she is kissed). Valentin says he is not the panther
woman but the spider woman, hence Kiss of the Spider Woman.