This association with signatures undoubtedly stems from the fact that John Hancock had the largest and most flamboyant signature on the Declaration of Independence. The legend goes that he wanted to be sure the English King (George III) could read it without his spectacles but others say John's signature was always that large.
http://en.wikipedia.org/wiki/John_Hancock
I wanted to list some of the others, and even though I expect most people would know them I thought I'd include links of the full historical reference--they're all interesting.
Waterloo was where Napoleon met his defeat, synonymous of course with an obstacle that is impossible to overcome, that defeats you. One person confused this with Watergate, then didn't know any more about Watergate than the name and association with Nixon and scandal.
http://en.wikipedia.org/wiki/Battle_of_Waterloo
Norman Rockwell was an artist who painted pictures of nice, normal American families, used to indicate same or idealized family.
http://www.normanrockwell.com/
Benedict Arnold Revolutionary War leader and, later on, a traitor, used for anyone who's a betrayer or treasonous.
http://en.wikipedia.org/wiki/Benedict_Arnold
McCarthyism, McCarthy (see link for full explanation) has come to be used against those who imply or state that someone is not patriotic or loyal to the country. Such statements are often made by intensely partisan or fanatical people towards those with an opposing view, but it initially referred only to anti-communism.
One person thought McCarthy might have been one of our early presidents!
Machiavellian has another complex origin, and time has once again distorted the meaning. Machiavelli was a politician and statesman during the Italian Renaissance (1469-1527), and he wrote books on his political ideas.
According to Wikipedia:
The pejorative term Machiavellian as it is used today (or anti-Machiavellism as it was used from the sixteenth century) is thus a misnomer, as it describes one who deceives and manipulates others for gain; whether the gain is personal or not is of no relevance, only that any actions taken are only important insofar as they affect the results. It fails to include some of the more moderating themes found in Machiavelli's works and the name is now associated with the extreme viewpoint.
Florence Nightingale (1820-1910) famous as a nurse who also wrote books about her profession, and also a statistician (useful in the study of epidemiology). Her name is now used to indicate someone who nurtures someone who is ill, often directed at family members and friends, such as "She's a regular Florence Nightingale."
http://www.florence-nightingale.co.uk/index.php
