expression, godan verb (-su)
close up like an oyster; retire into one's shell
Rare idiomatic expression meaning to shut oneself off from others, like a clam closing its shell. More common alternatives include 殻に閉じこもる.
彼は失恋してから、すっかり殻を閉ざしてしまった。
After his heartbreak, he completely closed himself off from others.
More common expression with the same meaning of withdrawing into one's shell.
From 殻 (shell) + を + 閉ざす (to shut). The image is of a clam or oyster closing its shell, metaphorically applied to a person shutting themselves off from the world.