[gerdesj]

"What nonsense"

That's an example of idiom. The word "what" is overloaded to mean "that is" - for dramatic emphasis.

The verb "to be" is often overloaded or implied in many languages. For example "quelle surprise" in French.

Perhaps we are dealing with a point of vocabulary. When I first learned Latin, we had "Civis Romanus" and "Mentor" as initial textbooks (kivvy was red and mental was blue). Later on a green book was added and it dealt with idiom (idia/idioms?) and I think that was its name, but I had moved on by then!