n. the posterior; one's butt end. (Because one roosts on it.) : Don't just sit there on your rooster. Get to work.
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
rooster
1772, from roost (earlier roost cock, 1606), in sense of "the roosting bird," favored in the U.S. as a puritan alternative to cock (and compare roach).
[roo-ster] 
stər