Discretion and chicken pot pie

I just finished copyediting a book about neuroanatomy, and the final chapter included instructions for assessing the patient's cognitive function. The examiner asks the patient to explain the meaning of the following proverbs, ranked from easy to difficult:

Don’t cry over spilled milk.

People who live in glass houses should not throw stones.

The tongue is the enemy of the neck.

One swallow does not a summer make.

You never cross the same stream twice.

I paused at the third, which I'd never heard before: "The tongue is the enemy of the neck." Huh?

My first association was that wartime slogan "Loose lips sink ships" -- meaning, watch what you say because you never know who might be listening. Good advice, surely (especially in a small town like ours). 

But what about the neck part? Perhaps it refers to medieval rulers who didn't hesitate to behead someone who said something that displeased (or, worse, disrespected) them.

I tried Googling the proverb and got nothing. Pity the poor patient sitting in a hospital bed having a doctor lob bizarre proverbs at him. And that was considered only the middle level of difficulty! To my mind, "You never cross the same stream twice," supposedly the pinnacle of complexity, was a lot easier to understand. 

The value of discretion was brought home to me in a ridiculous Facebook exchange. I belong to a group whose normally lighthearted members are fans of a particular mid-20th-century mystery writer. I posted a recipe for chicken pot pie that reminded me of one of the characters. My post received repeated and increasingly vitriolic feedback from a snarky Scotswoman who said it wasn't an authentic dish, it wouldn't have been baked by the character in question, and the administrator should delete my post.

I was itching to respond but for once held my tongue, suspecting that things would just escalate if I replied. And my patience paid off: the next day she got booted from the group.

Oh well!

Here's the recipe, BTW: Easy chicken pot pie! 1. Roast four chicken thighs (or use leftover chicken, or buy a cooked chicken from the store) and some diced potatoes. Take the meat off the bones. 2. Saute half an onion (chopped) and some chopped mushrooms in butter. When they are soft, add 1/3 C flour, salt and pepper, a 14.5-oz can of chicken broth, and 1/2 C milk and stir until nice and thick and bubbling. 3. Add the sauce, onions, and mushrooms to the chicken meat, the roasted potatoes, and your choice of vegetables (I used chopped carrots, green beans, and peas -- don't go overboard with quantity because all must fit into pie). 4. Line a 9-inch glass pie dish with a crust (I used store-brand refrigerated dough) and fill. Top with the second crust; seal and flute the edges. Cut vents and brush with egg wash. 5. Bake at 425 for about 35 minutes. Watch it toward the end so the edges don't burn. 6. Mine bubbled over a bit, so protect your oven floor.