Quiz: Remember the definition of mal de mer?
"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 (cf. Old High German werra "strife, conflict, war;" see war). Figurative use by 1861. As an adjective from 1811. Acquired by English during the Peninsular War (1808-1814); 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."