in one's shell

in one's shell or into one's shell  {adv.} or  {adj. phr.},  {informal}

In or into bashfulness; into silence; not sociable; unfriendly.

After Mary's mother scolded her, she went into her shell.

The teacher tried to get Rose to talk to her, but she stayed in her shell.

Contrast: OUT OF ONE'S SHELL.