What's the English idiom/saying to describe that the chosen word is not correct?

In some contexts, the word you're looking for is "understatement".

When done intentionally, it could be "irony".

Try this: "Big? That's an understatment!"

Irony: "I have a flight booked from NY to London. I get nervous flying over the big pond." (Atlantic Ocean)

Rather than It's not the word, try

That's hardly the word!

Amusingly, this idiomatic use of hardly itself suffers from the very same problem of understatement. But in this case, understatement in the English way of understatement, not the "big waves" way. You can use it almost anywhere:

• You realize you've left something unimportant behind: I'm hardly going to drive 100 miles just for that.

• Someone makes a feeble excuse: That's hardly the point.

'misnomer'--an inappropriate name for a thing or an inappropriate designation (Merriam-Webster).