theirs (thârz)pron. (used with a sing. or pl. verb)1. Used to indicate the one or ones belonging to them: The red house is theirs. If your car doesn’t start, take theirs.2. Usage Problem His or hers: brought his own lunch and expected everybody else to bring theirs. See Usage Notes at he1, they.[Middle English, from their, their; see their.]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.theirs (ð??z) pron1. something or someone belonging to or associated in some way with them: theirs is difficult. 2. not standard something or someone belonging to or associated in some way with an indefinite antecedent such as one, whoever, or anybody: everyone thinks theirs is best. 3. of theirs belonging to or associated with themCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014theirs (ð??rz) pron. 1. a form of the possessive case of they used as a predicate adjective, after a noun or without a noun: Are you a friend of theirs? It is theirs. 2. (used after an indefinite singular antecedent in place of the definite form his or hers): I have my book; does everyone else have theirs? 3. that which belongs to them: Theirs is the white house. [1150?1200] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.