Is it ensure, insure, or assure?
"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."