The "poison pill" was invented 40 years ago by business lawyer Martin Lipton to counteract a wave of hostile takeovers on Wall Street. "It was the age of the corporate raiders," Lipton explained to the media site The Deal in 2011, from investors such as Carl Icahn to Kirk Kerkorian. Quickly contested in court, the practice was declared legal for the first time in 1985 by the Delaware Supreme Court -- a tax friendly state where Twitter, although officially based in Californian, is incorporated.
