Read here more about it: http://www.hindu.com/edu/2009/11/24/stories/2009112450150402.htm
As children, when we misspelt a word, our teachers circled it, and made us rewrite it several times. Hoping that we would learn the correct spelling! When printers make an error in spelling, it sometimes results in the creation of a new word. Derring-do is one such example. The original expression as used by Chaucer was `dorrying don' meaning `daring to do'. Through a series of copying and printing errors, it became `derring-do'.
Read the actual story here: http://www.phrases.org.uk/meanings/derring-do.html
What is interesting to note is that a word can undergo such a change because of a printing mistake and thus getting accepted !