Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw Hill.
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Word Origin & History
rep
1705 as abbreviation of reputation; as a shortening of repetition it is recorded from 1864, originally school slang; as a shortening of representative, especially "sales representative," it is attested from 1896. As an abbreviation of repertory (company) it is recorded from 1925.
rep (rěp) n. Roentgen-equivalent-physical; a unit of absorbed radiation dose, equal to the amount of ionizing radiation that will transfer 93 ergs of energy to 1 gram of water or living tissue.