Q: In A Hero of France, Alan Furst’s latest World War II thriller, one of the characters uses the phrase “guerrilla warfare.” Did the word “guerrilla” really refer to an unconventional war in the early 1940s, when most of the novel takes place?
A: Yes, “guerrilla” has been used that way for more than 200 years, well before Alan Furst put the word into the mouth of Fabien, a sabotage instructor working with the Resistance in France.
The Oxford English Dictionary has examples dating from the early 1800s for “guerrilla” used as a noun or an adjective in reference to an unconventional war or someone fighting in such a war.
English borrowed the word directly from Spanish, where guerrilla is a diminutive of guerra, or war. The usual spelling in English is “guerrilla,” though some dictionaries also accept “guerilla.”
In the earliest OED citation, from an 1809 dispatch by the Duke of Wellington, the word refers to someone engaged in unconventional war: “I have recommended to the Junta to set … the Guerrillas to work towards Madrid.”
An 1811 citation, from The Vision of Don Roderick, a poem by Sir Walter Scott about Wellington’s victories in the Peninsula War, uses “guerrilla” adjectivally:
“But, with the darkness, the Guerilla band / Came like night’s tempest, and avenged the land.” (We’ve gone to the original and expanded the OED citation.)
An 1819 example, from an article in the Edinburgh Review by Sydney Smith, an Anglican cleric, uses “guerrilla” as a noun for the war itself, a usage that the dictionary describes as “now somewhat rare”:
“A succession of village guerillas;—an internecive war between the gamekeepers and marauders of game.”
Finally, the word “guerrilla” has been used loosely since the late 1800s to describe almost any kind of irregular, unorthodox, or spontaneous activity: “guerrilla advertising,” “guerrilla cooking,” “guerrilla filmmaking,” and so on.
The earliest OED example for the newer usage is from the November 1888 Issue of the Polyclinic, a medical journal in Philadelphia:
“The so-called pure pepsins … which, by a system of ‘guerrilla’ advertising … have been foisted upon the deceived medical profession.”
from Grammarphobia http://www.grammarphobia.com/blog/2016/07/guerrilla.html