Mammoth

It's not common; neither is it rare.

HUBLOTDo native English speakers commonly use "mammoth" as an adjective?

The word mammoth can be used as an adjective, in which case it means huge, e.g. a mammoth book.

HUBLOTDo you hear "a colossal mistake" more commonly than "a mammoth mistake"?

I do, but I don't hear either of them often. I mean I haven't heard either one in the last ten years. I consider them members of the set of "literary words". They are much more likely to be encountered while reading than while listening.

CJ

The origins of the two words are interesting. Colossus is one of those "inkhorn" terms, with origins in Latin and Greek. Mammoth is a more recent term:

colossus (n.) "gigantic statue," late 14c., from Latin colossus "a statue larger than life," from Greek. kolossos "gigantic statue," of unknown origin, used by Herodotus of giant Egyptian statues, and used by Romans of the bronze Apoll

There are only two adjectives in modern English, "awesome" and "massive".

In my view, "mammoth" is not at all uncommon as an adjective in the US today - I personally use it whenever I can, because I feel it gives great emphasis when trying to say something is big (I never use "colossal") - I like it's connection with the prehistoric animal, the wooly mammoth. Some examples of usage:

We've got a mammoth problem on our hands.

They arrived with this mamm

Thank you so much for your impressive responses, all.

The US has a colossal debt to repay. How to do it is one matter of debate between the two political parties.