What does that mean? Origins of common terms

The origins of some figures of speech are obvious—putting the cart before the horse, for instance. Others are a little more obscure. From Jeff Rovin’s book The Unbelievable Truth! (Signet) here’s a look at the explanations behind three common expressions:
• Pulling the wool over their eyes. In the 17th and 18th centuries, thieves and robbers would yank their victims’ wool wigs down over their eyes so they couldn’t see who was attacking them.

• Blackmail. In 16th-century England, mail meant “rent” or “tribute.” Debts that had to be paid in silver were called “whitemail.” A debt that could be paid in any other way—from livestock to property—was called “blackmail.” Because blackmail did not have a set value, the person collecting the debt could extort any amount or anything they wished from the debtor.

• Red tape. For centuries, it was British custom to seal important documents with red wax and red tape. Cutting through it was the only way to get at the documents and read them.
 
Get more information like this:
The Word Origin 2011 Day-to-Day Calendar
Question of Origins
Webster's New Explorer Dictionary of Word Origins
The Origins of English Words: A Discursive Dictionary of Indo-European Roots
The Complete Idiot's Guide to Weird Word Origins
Who Said That First? The curious origins of common words and phrases