in one's shell

Having retreated into one's mind or otherwise isolated oneself so as to avoid undesirable thoughts, situations, or interactions with people. After getting mugged, I went into my shell and didn't speak to anyone for nearly a week. I'm not good around large groups of people; I usually just go into my shell and wait until I can be alone. Don't go into your shell because you don't want an argument—talk to me and let's discuss the problem!

Also, into one's shell. In a quiet or withdrawn state. For example, Jim is extremely shy; if you try to get him to talk he immediately goes into his shell. This usage alludes to the shell as a protective covering and dates from about 1800, as does the antonym, out of one's shell, as in Once Anne is out of her shell she's very articulate. However, the same expression was also used from the 1500s on to denote being young and inexperienced, alluding to a baby bird that had not quite emerged from its shell.