#1 sub v. [substitute, stand in, fill in] be a substitute; "The young teacher had to substitute for the sick colleague"; "The skim milk substitutes for cream--we are on a strict diet". wn
#2 sub n. [bomber, grinder, hero, hero sandwich, hoagie, hoagy, Cuban sandwich, Italian sandwich, poor boy, submarine, submarine sandwich, torpedo, wedge, zep] 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. wn
#3 sub n. [submarine, pigboat, U-boat] a submersible warship usually armed with torpedoes. wn
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