* Mammoth: Means extremely large or huge.
* Synonym: A word with the same or nearly the same meaning.
* Opposite: A word with the completely opposite meaning.
Therefore, the opposite of a synonym for "mammoth" would be a word meaning extremely small or tiny.
Here are some examples:
* Mammoth (synonym): Colossal, gigantic, immense
* Opposite (of synonym): Minute, tiny, minuscule, infinitesimal