"natural elevation rising more or less abruptly and attaining a conspicuous height," 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" (from PIE root *men- (2) "to project").
Until 18c., applied to a fairly low elevation if it was prominent (such as Sussex Downs or the hills around Paris); compare hill (n.). As an adjective, "of or situated on a mountain," from late 14c.
Mountain dew "raw and inferior whiskey" is attested by 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 is recorded from 1839; mountain-climbing from 1836. Mountain laurel is from 1754; mountain-lion "puma" is from 1849, American English; the mountain goat of the Western U.S. is so called by 1841 (by 1827 as Rocky Mountain goat).
word-forming element making adjectives from nouns, meaning "having, full of, having to do with, doing, inclined to," from Old French -ous, -eux, from Latin -osus (compare -ose (1)). In chemistry, "having a lower valence than forms expressed in -ic."
<a href="https://www.etymonline.com/word/mountainous">Etymology of mountainous by etymonline</a>
Harper, D. (n.d.). Etymology of mountainous. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/mountainous