IRRATIONAL EXUBERANCE

"'Irrational exuberance'" is a phrase used by former Federal Reserve Board Chairman Alan Greenspan in a speech given at the American Enterprise Institute during the stock market boom of the 1990s. The phrase was interpreted by financial pundits as a typically cryptic warning that the market might be overvalued.
Although it is sometimes believed that Greenspan's comment was made near the height of the dot-com boom (and contributed to its downfall), it was actually said much earlier, in December 1996 (emphasis added in excerpt):
The presence of the short comment—not repeated by Greenspan since—within a rather dry and complex speech would not normally have been so memorable; however it was followed by immediate slumps in stock markets worldwide, provoking a strong reaction in financial circles and making its way into common parlance. Greenspan's comment was well remembered although few heeded the "warning."
The losses were quickly recouped and eclipsed by the accelerating stock market boom; as of mid-2007 stock prices have never again fallen to the levels seen following the warning. The phrase was notably used by Yale professor Robert Shiller, who titled his book with it in 2000. When the market slumped in the same year, in response to increasing interest rates, the phrase got another popularity boost and was much used in hindsight, to characterize the excesses of the bygone era. In 2005, upon Greenspan's retirement from the Treasury Board, ''The Daily Show with Jon Stewart'' held a full-length farewell show in his honor, named "An Irrationally Exuberant Tribute to Alan Greenspan."
It had become a catch phrase of the boom to such an extent that, during the economic recession that followed the stock market collapse of 2000, bumper stickers reading "I want to be irrationally exuberant again" were sighted in Silicon Valley and elsewhere.