devil of it

devil of it or heck of it  {n. phr.}

1. The worst or most unlucky thing about a trouble or accident; the part that is most regrettable.

Andy lost his notebook, and the devil of it was that the notebook contained all his homework for the coming week.

When I had a flat tire, the devil of it was that my spare tire was flat too.

2. Fun from doing mischief. — Used after "for".