guerilla: [19] Etymologically, a guerilla is a ‘little war’. English acquired the word during the Peninsular War (1808–14) from Spanish guerrilla, which is a diminutive form of guerra ‘war’ (a word ultimately of Germanic origin, related to English war). In Spanish it still means ‘skirmish’, and until well into the 19th century it was used in English for a ‘war characterized by irregular skirmishing’ (the famous Times war correspondent William Russell, for instance, reported on 18 March 1862 that ‘Arkansas is now the theatre of a large guerilla’).
The first recorded use of the word in its present-day sense is by the Duke of Wellington, the British military commander in Spain, in a despatch of 1809: ‘I have recommended to the Junta to set the Guerillas to work towards Madrid’. => war
