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 necessarily constitute 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.
A term used to describe anything thats very tight, in a satirical manner, because nuns can't have sex, therefore there cunts would be tight. plus its fun to say, and it flows nice
Danm bro this fucking nail is tighter then a nun's cunt