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1640s, from sub- + marine (adj.).
"submarine boat," 1899, from submarine (adj.). The short form sub is first recorded 1917. Submarine sandwich (1955) so called from the shape of the roll.
A substitute of any sort, esp an athlete who replaces another or an athlete not on the first team (1830+)
: Who'll sub for me when I go on leave? (1853+)