A suet pudding is a boiled, steamed or baked pudding made with wheat flour and suet (raw, hard fat of beef or mutton found around the kidneys), often with breadcrumb, dried fruits such as raisins, other preserved fruits, and spices.
Mary Kettilby's 1714 A Collection of above Three Hundred Receipts in Cookery, Physick and Surgery gives a recipe for "An excellent Plumb-Pudding", which calls for "one pound of Suet, shred very small and sifted" along with raisins, flour, sugar, eggs, and a little salt; these were to be boiled for "four hours at least".
The ancestor of the suet pudding was pottage, a meat and vegetable stew originating in Roman times.
This was prepared in a large cauldron, the ingredients being slow cooked, with dried fruits, sugar and spices added.
In the 15th century, plum pottage was a mixture of meat, vegetables and fruit served at the beginning of a meal.