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.
1 Cold to the point of being painful to touch. Associated with cold and hard as ice.
2 An object that cannot be made warm
3 An inanimate object devoid of feeling or desire.
Very blunt - referring to the fact that all nuns must vow to celibacy, and thus are not allowed to have sex. However, a lack of sex does not necessarilyconstitute a "bluntness" of the vagina (in fact, a blunt vagina would definitely be preferable to a sharp one, for ease of use), so the origin of the phrase likely originates from somebody looking for a funny sounding dirty rhyme.