What kind of literary technique is "living behind closed doors,"?

I think it is an idiom but I might be wrong

Explanation:its an idiom

malvolio only loves olivia because he thinks if they get married to olivia he will have a noble title, people to serve him, and fine clothes to wear. also he thinks if they get married he will have many expensive things, and he thinks he will be able to order toby around. malvolio only wants to be noble and boss people around he dose not really love olivia. but viola love for duke orsino is real she really loves orsino.

i think he will fail because he tried to get her to love him and she said oh, you are sick of self-love, malvolio. there is no slander in an allowed fool. so i think she knows what malvolio is trying to do. she knows that he only wants to marry her because he wants to be noble and boss people around so that it why i think she said oh, you are sick of self-love, malvolio. there is no slander in an allowed fool because she new he was selfish.