Nun's finger 

Nun's finger refers to the lump on the middle finger from writing long papers with a pen. It's common among university students during their exam periods, especially MBA and medical students who have very long exams papers to write. It is thought to have come from a joke about nuns getting a similar lump, or rubbing sore from holding candles in a similar way to a pen but using them to pleasure themselves. There is also possible reference to the virginity of a finger, as the nun's finger syndrome is more common when the finger owner has not performed a great deal of intensive writing for some time.

As I am a computer programmer, I don't write with a pen any more I have a nun's finger from all my MBA exams.

Nun's finger by Mother superiority August 2, 2016