SOS is shorthand for a dish common to U.S. Army mess halls. The polite version (long-form) is Stew on a Shingle.

The item below was sourced from Urban Dictionary:

My dad was in the army for 24 years and my mom use to make this for him. I love the stuff. The navy uses chipped beef but I prefer the hamburger version.

Here is an official U.S. Army recipe for SOS:

CREAMED BEEF ON TOAST (SOS)
1/2 lb. ground beef
1/4 tsp. salt
1/4 tsp. pepper
4 tbsp. sifted flour
1 cup evaporated milk
1 cup water
2 tbsp. butter

Brown ground beef in its own fat. Remove excess fat and save for making roux. Season with salt and pepper. To make roux, place 2 tbsp. reserved fat in double broiler or heavy pan. Slowly add sifted flour, stirring constantly over low heat until thoroughly blended. Cook for five minutes. Do not brown. Combine milk and water. Add butter and scald (not burn) in double broiler or heavy pan. Add roux to scalded milk, stirring constantly until thoroughly blended. Add meat mixture and cook about 10 minutes, or until desired consistency. Serve on toast.