Meaning

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 U

noun a submersible warship usually armed with torpedoes

verb be a substitute

origin* Shortened form of any of various words beginning with sub-.

Etymology from Wiktionary, CC BY-SA 3.0