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!