What is the maculine gender of nun?

A Male nun is more commonly known as a non existent thing. So they don't exist

Roman Catholic AnswerPerhaps you are thinking of a monk. Sisters are the most visible representatives of the religious life, often portrayed in film and such. Brothers are not nearly as famous, as their life is not as common in the everyday culture and entertainment.

from Modern Catholic Dictionary by John A. Hardon, S.J. Doubleday & Co., Inc. Garden City, NY 1980Monk.Originally a hermit or anchorite, but already in the early Church applied to men living a community life in a monastery, under vows of poverty, chastity, and obedience, according to a specific rule, such as that of St. Basil or St. Benedict. (Etym. Greek monachos, living alone, solitary.)