The expression “the fat is in the fire” means: the mischief is done and unpleasant results must be faced; an irretrievable blunder has been made and ill consequences will follow; some dire act has been committed which will undoubtedly provoke an explosion of anger.
Nobody knows how old this saying may be, and the beginning can only be guessed.
It was recorded in John Heywood’s A Dialogue Conteynyng Prouerbes and Epigrammes (1562), and, accordingly, must have long been in use before that time.
But in those days the saw meant that some project had failed and one must cut his cloth accordingly. Heywood wrote, “Than (Then) farewell riches, the fat is in the fire.”
One may surmise, probably correctly, that the original allusion was to a chunk of fat meat which, thrust through by a spit on the hearth to roast, caught ablaze and fell into the fire to the dismay of the cook.