sub (n.)

shortened form of substitute (n.), 1830; the verb in this sense is from 1853. Related: Subbed; subbing. From 1917 as short for submarine (n.).

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Definitions of sub from WordNet
sub (n.)
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
sub (n.)
a submersible warship usually armed with torpedoes;
Synonyms: submarine / pigboat / U-boat
sub (v.)
be a substitute;
Synonyms: substitute / stand in / fill in