1. To retreat into one's mind or otherwise isolate 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!
2. To act in a reserved and/or defensive manner. The team went into their shells after half time, trying to protect a slim lead.The senator went into his shell when reporters tried to question him about allegations of tax fraud.