To take a customary action to ward off some misfortune that is believed to be attracted by a presumptuous statement.
Hopefully; a self-directive to undertake the customary action to ward off bad luck.
To assume something to be true without verification or proof.
To give little attention to or to underestimate the value of; to fail to appreciate, especially something one has grown heavily accustomed to; not to value or care for appropriately, out of the assumption that the person or thing not valued cannot be lost.