The English word flummery comes from the Welsh llymru. During the 1600s, the popularity of this cold oatmeal pudding spread to the English counties, and versions of it were famously eaten in Cheshire and Lancashire.
In Wales, llymru still denotes a specific dish: oatmeal boiled until it reached a jelly-like consistency, and served cold as a pudding. But by the time our Cookbook was being compiled in the Eighteenth Century, the English were using flummery to describe other jellied desserts, such as blancmanges and semi-set creams:
Halfe an ounce of isinglass disolved in half a pinte of boiling water. When it is cold, put to it half a pint of white wine, the yolks of four eggs well beaten, the peel of a lemon and the juice of one and a halfe. Sweeten it to your taste, put it on the fire. Make it hot, just ready to boile. If you let it boil, it will be apt to curdell. Then run it through a cloath into a beason. Stir it whin cold. Eat in your cups. In sumner add a littell more isinglass.
Take the jelly of 2 calves feet skimd clean from fat. Put to it a qr of a pd of almonds that are very sound pounded very small with rose water, the yallow rind of a lemon cut thin, a stick of cinnimon, 3 spoonfull[s] of orange flower water, some sugar to yr taste. Let these boyle 4 or 5 times together, then put in half a pint of good cream. Let it get a boyle or 2 in it, then take it of & strain it through a muslin. Stir it in a bason continually till it is cold.