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Used as an exclamation of pleasure, approval, elation, or victory.
[Alteration of yea.]
yay 2or yea (yā)
So; by this amount: "The fish was yay big" he said, spreading his arms three feet apart.
[Probably from yea.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
informal an exclamation indicating approval, congratulation, or triumph
[C20: perhaps from yeah]
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
yay(yeɪ) Informal. interj.
2. so; to this extent: I knew you when you were yay high.
[1960–65; probably alter. of yea]
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.