When you make a really, really big mistake, this is an example of a mistake that would be described as a mammoth mistake.
A large extinct furry elephant type animal is an example of a wooly mammoth.
Origin of mammoth
- Ultimately (perhaps influenced by behemoth) from Russian mamont, mamot probably from earlier Mansi (Ugric language of western Siberia) mān-oŋt- mān earth (akin to modern Mansi mā- earth) (as in mā-xar mammoth (literally, “earth stag”)) oŋt- horn (in reference to fossil mammoth tusks)
From American Heritage Dictionary of the English Language, 5th Edition