Real Dictionary

mammoth
You can use mammoth to emphasize that a task or change is very large and needs a lot of effort to achieve.
countable noun: A mammoth was an animal like an elephant, with very long tusks and long hair, that lived a long time ago but no longer exists.

mammoth in American English
any of a genus (Mammuthus) of extinct elephants with a hairy skin and long tusks curving upward: remains have been found in North America, Europe, and Asia
very big; huge; enormous
adjective: immensely large; huge; enormous
noun: any large, elephantlike mammal of the extinct genus Mammuthus, from the Pleistocene Epoch, having hairy skin and ridged molar teeth

mammoth in British English
adjective: of gigantic size or importance
noun: any large extinct elephant of the Pleistocene genus Mammuthus (or Elephas), such as M. primigenius ( woolly mammoth), having a hairy coat and long curved tusks