The definition of foul play is a treacherous action, unfair play or play that breaks the rules of the game.
When a fire is caused by arson, this is an example of a situation when there is foul play.
When someone tackles in football in a way that is against the rules, this is an example of foul play.
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unfair play; action that breaks the rules of the game treacherous action or violence; esp., murder Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
Unfair or treacherous action, especially when involving violence.
The American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved. foul play - Phrases/Idioms
Unfair or treacherous action, especially involving violence. For example,
The police suspected he had met with foul play. This term originally was and still is applied to unfair conduct in a sport or game and was being used figuratively by the late 1500s. Shakespeare used it in The Tempest (1:2): “What foul play had we, that we came from thence?” The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.