Lady Capulet is being unintentionally ironic in her choice of metaphor. She says that Juliet is “mewed up” — confined to her room — due to her sorrow. The term “mewed up” comes from falconry. During the molting process when the falcon is shedding its old feathers and growing new ones, it can’t fly and is therefore defenseless. The owner mews up the falcon in a cage to protect it from predators. Here Juliet is mewed up where Paris cannot get to her.