Meaning

noun any of numerous extinct elephants widely distributed in the Pleistocene; extremely large with hairy coats and long upcurved tusks

adjective so exceedingly large or extensive as to suggest a giant or mammoth

originFrom obsolete Russian ма́мант (mámant), modern ма́монт (mámont), probably from a Uralic word, such as Proto-Mansi *mān-oŋt (“earth horn”). Possibly influenced by behemoth. Adjectival use was popularized in the early 1800s by references to the Cheshire Mammoth Cheese presented to American paleontologist and president Thomas Jefferson.

Etymology from Wiktionary, CC BY-SA 3.0