in one's shell

Idioms and Phrases

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 In a quiet or withdrawn state.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.