The word "mew" derives from the Latin word mūtāre, to change, which also gives us our English word mutate. But mew is only used in reference to a particular type of change: the molting of a bird when it loses its old feathers and grows new ones. Because a bird can’t fly during this process, the owner of a valuable hunting falcon would keep it “mewed” up, confined in a protective enclosure where it would be safe from predators. 

Here, Theseus is using mew in a figurative sense to describe nuns who are mewed up in a convent to protect them from male predators.