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2.
Something added to a situation, company, deal, object, etc., to make it undesirable to takers, buyers, thieves, investors, or conquerors.
"They're planning a hostile takeover of our company, but we're taking on a huge debt which ought to serve as a poison pill and turn them off."
by Anarcissie May 17, 2008
 
1.
The actions done by a party to screw another party after a major change has been completed or in the mix.
John Doe the outstanding CEO of ABC company resigned from his job today due to a takeover bid from XYZ. This will most likey lower the stock price of the company and make it undervalued vs. XYZ's bid. John Doe's Poison Pill will make make XYZ's bid overvalued.
by John Willikins December 10, 2005