The Phrase Finder is an useful site you can use to find “the meanings and origins of over 1,200 English sayings, phrases and idioms,” according to its web site.  There are detailed explanations providing the origins of all sorts of sayings.  There is a search function of course, but there’s also a  Phrase of the Week newsletter to which you can subscribe.  Some recent posts include Donkey’s years, Toodle-oo, Think outside the box, With bells on, Chop-chop and Silver bullet.

Sources used for the research include primary sources such as newspapers, books, etc., and “trusted references” such as  The Oxford English Dictionary, Second Edition and The Historial Dictionary of American Slang, First Edition, according to the web site.

Thanks to David King, who wrote about it in an article published in the July/August 2008 Information Today, Fun Site Traces Origins of “Riding Shotgun,” p. 37.