go into one's shell

From turtles, who go into their shells to protect themselves.

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go into one's shell

1. (idiomatic) To act defensively.
   • 2011 December 7, David Ornstein, “FC Basel 2 - 1 Man Utd”, in BBC Sport‎:

     Instead of gathering the ball, De Gea poked it away with his foot and Streller was able to place a powerful half-volley into the net.
     United went into their shells and it was not until the 29th minute that they mustered a response - one from which they should have equalised.