Poison pill originally meant a literal poison pill (often a glass vial of cyanide salts) carried by various spies throughout history, and by Nazi leaders in WWII. Spies could take such pills when discovered, eliminating any possibility that they could be interrogated for the enemy's gain. It has since become a term referring to any strategy, generally in business or politics, to increase the likelihood of negative results over positive ones for anyone who attempts any kind of takeover.