come out of one's shell

Example: “Joining the drama club really helped her come out of her shell.”

Origin: Draws on an animal that retreats into its own shell for protection whenever it senses danger — a turtle, most obviously, though the 19th-century writer Ebenezer Cobham Brewer suggested the image could just as easily be a chick pecking its way out of an egg for the first time. Either way, the shell stands in for whatever protective barrier keeps a shy or guarded person separated from everyone around them, and "coming out" of it describes the deliberate, sometimes difficult act of dropping that barrier and engaging with others directly.