"fighter in an irregular, independent armed force," 1809, from Spanish guerrilla "body of skirmishers, skirmishing warfare," literally "little war," diminutive of guerra "war," from a Germanic source cognate with Old High German werra "strife, conflict, war," from Proto-Germanic *werra- (see war (n.)). Acquired by English during the Peninsular War (1808-1814), when bands of Spanish peasants and shepherds annoyed the occupying French. Purists failed in their attempt to keep this word restricted to "irregular warfare" and prevent it taking on the sense properly belonging to guerrillero "guerrilla fighter." Figurative use by 1861. As an adjective from 1811.