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general::
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 In a quiet or withdrawn state.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.
American Heritage Idioms