If you're in the Navy, a submarine is a military ship that goes under the water. If you're hungry, it's a type of sandwich.
This word mainly refers to a vessel that travels underwater and is armed with torpedoes — also known as a U-boat. But you can submarine someone by attacking them with a submarine or bringing them down with a blow to the legs. A disappointment can submarine — or sink — your hopes and dreams. In the kitchen, a submarine is what you call a long, crusty sandwich filled with meats and cheese — also known as a sub, grinder, hero, or hoagie.