What is the origin of the American expression "s*** fire"?

Solution 1:

The earliest reference in OED is from 1598, from John Florio's Italian/English dictionary A worlde of wordes:

Cacafuoco, a hot violent fellow, a shite-fire.

We can say that the term was coined by J.Florio. The books An Encyclopedia of Swearing (by Geoffrey Hughes) and Joy of Swearing (by M. Hunt, Alison Maloney) support this claim also.

John Florio was born in London and he was of Anglo-Italian origin. Thus, the word shitfire is of British origin.

Interesting note: Cacafuego is a rarely used loan-word in English also and there is an entry of the word in OED where it is defined as:

A spitfire; a braggart.
   _{(The name of the Spanish galleon taken by Drake in 1577.)}

Here is the etymology from OED:

< Latin cacāre, Spanish cagar, Portuguese cagar to discharge excrement + Spanish fuego (Portuguese fogo) fire < Latin focus hearth.

Solution 2:

From A Garden of Words by Martha Barnette:

• The Spanish word cacafuego is usually defined as "a braggart" or "spitfire," but its literal meaning is "shit-fire." This expression became somewhat popular in England after Sir Francis Drake captured a ship of the same name.

The term is of Spanish origin, a literal,translation of cacafuego which first appears in writing in the Italian English dictionary A Worlde of Wordes by John Florio (1553–1625) (who was born in London), a linguist and lexicographer, a royal language tutor at the Court of James I, and a possible friend and influence on William Shakespeare.

• This word, which means “braggart,” is Spanish in origin,literally translating as “shitfire.” It was the nickname of a ship captured by the Pirate Sir Francis Drake, who is presumably the braggart referenced by the word.

From (wordorigins.com):

• I don’t think Drake is being referenced as a braggart. The nickname of the Nuestra Señora de la Concepción was an obvious metaphor for her armament of guns, then quite a new development in maritime warfare.

• Also, the first (1598, which considerably pre-dates the first for cacafuego) citation for shitfire in the OED comes from John Florio’s Italian dictionary where it is given as the translation not for the Spanish but for the Italian cacafuoco.

• Of course, given that so much of Italy was under Spanish domination at the time, so much so that Spanish was the language of officialdom and the elite in several Italian states, it’s moot which language borrowed it from which, or whether they each happened on it independently. But if both the Spanish and Italian versions were known in England at the time, it’s no surprise that it was the Spanish one that stuck - the ‘aggressive, boastful Spaniard’ was, for good reasons, a figure far more present to the English mind than the ‘aggressive, boastful Italian’.

Note:

• Spitfire appears in English at about the same time as shitfire. I don’t suppose it’s possible to tell whether this was a euphemism for it, or a genuinely independent metaphor. (After all, if you have a clean and literal mind, when you face someone and hurl defiance, anger or cannonballs directly at them, it’s a more logical metaphor to say that you spit fire rather than shitting it.