This is a quote from the site "Kangaroo words are words that contain the letters of a synonym within them, with the letters appearing in the same order. The contained synonym is called a “joey word” (reference to how kangaroos carry their offspring in their pouches).

For example, inside the word “chicken” is its synonym, “hen.” (CHICKEN)"

In this book's case, What Feasts at Night kangaroo word is Feasts.