Sub is short for submarine, subscription, substitute or a submarine sandwich.(noun)
To sub is to substitute, to use someone or something instead of the person or object normally used.(verb)
An example of sub is to put in a different player when a player gets injured.
See sub in Webster's New World College Dictionary
Origin: < L sub, under, below: see up
See sub in American Heritage Dictionary 4
Origin: Middle English
Origin: , from Latin
Origin: , from sub, under; see upo in Indo-European roots.
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