noun
a string used in a musical instrument, such as a guitar or violin, that is the third in order of importance or thickness
adjective
referring to the position or order of the string, such as being the third in a series
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The third string quarterback was called in to play after both of the starters were injured.
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They had to bring in the third string player because the first two were out.
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The coach decided to put in the third stringer to see what he could do.
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In the company, she was considered the third string employee until she proved herself.