sub

(noun) a large sandwich made of a long crusty roll split lengthwise and filled with meats and cheese (and tomato and onion and lettuce and condiments); different names are used in different sections of the United States. Synonyms: bomber, grinder, hero, hero sandwich, hoagie, hoagy, Cuban sandwich, Italian sandwich, poor boy, submarine, submarine sandwich, torpedo, wedge, zep.

(noun) a submersible warship usually armed with torpedoes. Synonyms: submarine, pigboat, U-boat.

(verb) be a substitute. Synonyms: substitute, stand in, fill in. Examples: "The young teacher had to substitute for the sick colleague." "The skim milk substitutes for cream--we are on a strict diet."