Nucatum is a Vulgar Latin term for a dessert made with walnuts, honey, and egg white. Various webpages claim that a recipe for it is recorded in Apicius' de re culinaria, but I have not been able to find any such recipe in any copy of Apicius. A recipe for nucato is found in Anonimo Toscano, Libro della Cocina, a manuscript from the late 14th to early 15th century, translated by Ariane Helou (link below).
She translates the 14th century recipe as follows:
Honey boiled with walnuts, called nucato.
Take boiled and skimmed honey, with walnuts chopped slightly and spices, cooked together; dip your hands in water and spread it out; let it cool and serve it. And you can use almonds and hazelnuts in place of walnuts.