Definitions of 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
• noun:   a submersible warship usually armed with torpedoes
• verb:   be a substitute

  Example: "The young teacher had to substitute for the sick colleague"

• name:  A surname (very rare: popularity rank in the U.S.: #67417)