Is it ensure, insure, or assure?
c.1200, from Old French montaigne (Modern French montagne), from Vulgar Latin *montanea "mountain, mountain region," noun use of fem. of *montaneus "of a mountain, mountainous," from Latin montanus "mountainous, of mountains," from mons (genitive montis) "mountain" (see mount (n.)).
Until 18c., applied to a fairly low elevation if it was prominent (e.g. Sussex Downs, the hills around Paris). As an adjective from late 14c. Mountain dew "raw and inferior whiskey" first recorded 1839; earlier a type of Scotch whiskey (1816); Jamieson's 1825 "Supplement" to his Scottish dictionary defines it specifically as "A cant term for Highland whisky that has paid no duty." Mountain-climber recorded from 1839; mountain-climbing from 1836.
A generally massive and usually steep-sided, raised portion of the Earth's surface. Mountains can occur as single peaks or as part of a long chain. They can form through volcanic activity, by erosion, or by uplift of the continental crust when two tectonic plates collide. The Himalayas, which are the highest mountains in the world, were formed when the plate carrying the landmass of India collided with the plate carrying the landmass of China.