A world of words
Guerrilla , as modifier, means referring to actions or activities performed in an impromptu way, often without authorization.
This is one of the few words English has borrowed from Spanish.It dates back to early 19th and it was introduced during the Peninsular War (“the campaign waged on the Iberian peninsula between the French and the British, the latter assisted by Spanish and Portuguese forces, from 1808 to 1814 during the Napoleonic Wars. The French were finally driven back over the Pyrenees in an expedition led by Wellington”OD).
Londoners wonder if the guerrilla stickers which have recently appeared on the city underground “is a new form of vandalism or a rebellion against too many signs. So-called guerrilla stickers have been popping up on the Tube network. A professor at the London School of Economics believes it is a reaction from some passengers fed up of being treated like children.” (BBC News)