Explanation of "harder than a monk's jockstrap"

In Hard Girls by Martina Cole there is this sentence (it's on Google books if you want context):

Patrick had once remarked that Jennifer James was harder than a monk's jockstrap

I'm a native English speaker, so I have some idea what the author is getting at. But the context implies hard as in tough, and the phrase implies hard as in hard-to-get.

I would really appreciate an explanation of that phrase!

Solution 1:

It means "since nothing gets in and nothing can (no matter how much it might want to) get out, it must be as hard as adamantium". If Jennifer Jones is harder than that she must be one tough broad.