expression, godan verb (-ru)
withdraw into oneself; shut oneself away; seclude oneself
Idiomatic expression literally meaning 'to shut oneself inside a shell', used figuratively for emotional withdrawal or isolating oneself from others.
失恋してから、彼は殻に閉じこもってしまった。
After the heartbreak, he withdrew into himself.
新しい環境に慣れず、殻に閉じこもりがちになる。
Unable to adjust to the new environment, she tends to shut herself away.