Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Online Etymology Dictionary, © 2010 Douglas Harper
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The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition.
Copyright 2007. Published by McGraw Hill.
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The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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