Green’s Dictionary of Slang

1. a phr. used to indicate that a plan has failed.

c.1698		B.E. Dict. Canting Crew n.p.: All the Fat is in the Fire, of a miscarriage or shrewd turn.
1772		Bridges Burlesque Homer (3rd edn) 368: For heathen Gods, if you’ll inquire, / Are pleas’d when all the fat’s i’th fire.
c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Fat. All the fat is in the fire; i.e., it is all over with us: a saying used in case of any Miscarriage or Disappointment in an Undertaking, an Allusion to overturning the Frying pan into the fire.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn).
1797		Bridges Burlesque Homer (4th edn) II 151: [as cit. 1772].
1811		Lex. Balatronicum.
a.1848		N.Y. Herald n.p.: But take care that you don’t, like the Paddy, touch off your machine at the wrong end; for [...] then the fat would be in the fire, and you would be where the devil could give more reliable information about you than any other of your near relations [B].
1871		Sporting Gaz. (London) 11 Feb. 11/3: The fat’s in the fire, all’s exploded at last.
1918		Kia Ora Coo-ee 15 Apr. 2/3: Well, the fat was in the fire then, I saw trouble looming ahead, when Three Stars said, ‘Hurry up, rip the pelt off, and let’s have half of it.’.
1928		M.C. Sharpe Chicago May (1929) 122: Once they had him, the fat was in the fire.
1952		W.R. Burnett Little Men, Big World 221: If you do nothing yourself there is no danger of you making mistakes. But now the fat was in the fire.

2. a phr. used to indicate that the result of an action will be to provoke anger.

1887		‘’Arry in the Witness-Box’ in Punch 5 Feb. 61/2: ‘You ask me another,’ sez I. / Then, Oh, wasn’t the fat in the fire, Charlie? Wigging? That isn’t the word.
1900		Bulletin (Sydney) 6 Oct. 25/3: Another letter from the legal manager contained a lovely series of questions and suggestions for answers re the company’s properties and their prospects. But the officials didn’t bite, and Parliament got hold of the documents, and all the fat was in the fire.
1908		‘Doss Chiderdoss’ ‘Outside and Declined’ Sporting Times 8 Aug. 1/3: If any other man lands out, the fat is in the fire, / He and I are pretty sure to be colliders.
1920		Marvel 28 Aug. 10: Phew! The fat is in the fire this time!
1930		(con. 1900s–10s) Dos Passos 42nd Parallel in USA (1966) 258 The fat’s in the fire in this country now.
1951		Cusack & James Come in Spinner (1960) 323: It would only need Claire to complain about appointments in private rooms during Salon hours and the fat would be in the fire.
1969		D. Pendleton Executioner (1973) 113: Bolan’s (ha-ha) Indian had pulled the twenty-four-hour cop’s fat out of the fire. For the moment, at any rate.
2001		K. Waterhouse Soho 195: That editress of his on the Examiner would have his guts for garters. Alex could almost smell the stench of fat sizzling in the fire.

3. a phr. used to describe an energetic situation, but with no negative overtones.

1953		A.J. Liebling Honest Rainmaker (1991) 114: The fat is in the fire, with Mr. Brolaski dilating [...] upon the opportunity [...] whereby we two could now make a whole ton of money.
1964		L. Auchincloss Rector of Justin 96: When Eliza discovered one night that her handsome dinner partner was a Balliol man and a friend of Horace Havistock’s, the fat was in the fire.
2012		S. Blass Pirate for Life 173: So now we had a 2-0 game, runners at second and third, just one out, and the whole thing was in the fire.