tache d’huile
n.— «In these facts it is to read a prediction of the ultimate Germanizing of the whole of that peninsula.…To use a common French expression, the German race fait tache d’huile; it spreads and spreads, and gradually covers the surface upon which it at first was but a spot.» —“Prussia” Times (London, England) June 8, 1864. (source: Double-Tongued Dictionary)