In bocca al lupo and why is it "al"?

Why is it in bocca al lupo and not in bocca del lupo? Do we use "al" instead of "del" for other body parts also? is this just for this phrase specifically?

Every article about it explains the good luck thing, but they all say it means "in the mouth of the wolf" and I swear that should be "in bocca del lupo" if it were literal. Help me understand!