What's the opposite of opposite?

From National Review Online:

A reader writes:

Mr. Derbyshire,

In your July Diary on NRO you mentioned your annoyance with words that have no opposite. Earlier today I was discussing languages with a friend and I recalled that in the past I have used the word “shallow” as a sort of vague, inexact opposite to “steep”, but it never seemed right. What is the actual opposite of “steep”? What word can you put in place of “steep” in “this mountain is very steep” to make it mean the opposite? I don’t think there is one.

Just so. We need an adjective to describe words like this — words with no opposite. “Anantonymous”?

But then … what would be the opposite?