‘The fat is in the fire’ is used to express the opinion that a plan has gone irretrievably wrong, because of some calamity.

This proverbial saying is first found in John Heywood’s 1546 glossary A dialogue conteinyng the nomber in effect of all the prouerbes in the englishe tongue:

Nowe if we awarde me this wydowe to wedde,
And that I dryue of tyme, tyll tyme she be dedde:
Than farewell ryches, the fat is in the fyre.
And neuer shall I to lyke riches aspire.