n. a tough old thing, person, or idea. : What time does the old warhorse's train get in, and how long is she staying this time?
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
warhorse
1653, "powerful horse ridden into war;" the fig. sense of "seasoned veteran" of anything is attested from 1837.