montagnes russes is feminine in French.
rollercoaster (amusement ride); rollercoaster (any situation in which there are ups and downs or violent changes; hectic situation)
“2010, Paulo Coelho, Onze minutes (original title: Onze minutos), Flammarion (publ.). Je suis restée très longtemps devant les montagnes russes : je voyais que la plupart des gens entraient là en quête d'émotions mais, une fois que les véhicules se mettaient en marche, ils mouraient de peur et suppliaient qu'on les arrête.” — I stood for a long time by the roller coaster, and I noticed that most people get on it in search of excitement, but that once it starts, they are terrified and want the cars to stop.
Literally, “Russian mountains”, a predecessor to the modern roller coaster popular in 18th century Russia and introduced in Paris in 1812 under the name Les Montagnes Russes à Belleville.