Etymology

Guerrilla, from the Spanish term guerra, or War, with the -illa ending diminutive, could be translated as small war. The Spanish word itself, may have its origins in the early Germanic word werra or perhaps the middle Dutch word warre. They were adopted in Hispania around the 5th century by the Visigoths. The use of the diminutive is probably to evoke the difference in size between the guerrilla army and the state army against which they fight. The term was invented in Spain to describe the tactics used to resist the French regime instituted by Napoleon Bonaparte. Its meaning was soon broadened to refer to any similar resistance of any time or place. The Spanish word for guerrilla fighter is guerrillero. According to the OED, the term guerrilla was used in English as early as 1809 to describe the persons involved rather than just the tactics. In most languages the word still denotes the specific style of warfare. However, this is changing under the influence of broad English usage.

http://en.wikipedia.org/wiki/Guerrilla_warfare