
To go, retreat, etc. into your shell
Emotional Trigger
The phrase is used when someone is upset or embarrassed and becomes very quiet and less communicative.

He's been quiet all day; I think the criticism made him retreat into his shell.
Temporary State
It implies a temporary state, suggesting the person may come out of their shell eventually.

She's gone into her shell right now, but she'll be her usual self with some time.
Not Literal
This phrase is figurative and does not refer to physically hiding within a shell like a turtle might do.

Don't worry, he hasn't literally disappeared, he's just gone into his shell for a bit.
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