roost·er (ro?o?st?r)n.1. a. An adult male chicken.b. An adult male of certain other birds.2. A person regarded as cocky or pugnacious.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.rooster (?ru?st?) n (Animals) chiefly US and Canadian the male of the domestic fowl; a cockCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014roost?er (?ru st?r) n. 1. the male of domestic fowl and certain game birds; cock. 2. Informal. a cocky person. [1765?75; roost + -er1] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.