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 crawled 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 crawl into my shell and wait until I can be alone. Don't crawl into your shell because you don't want an argument—talk to me and let's discuss the problem!