In Middle English, the word pygg referred to a type of clay used in pottery.  People often kept their money in jars made out of this clay.  They were called “pygg jars.”  Eventually this evolved into “pig bank,” and finally “piggy bank.”  I remember seeing in cartoons people breaking their piggy banks open with a hammer to get the money.  I always questioned the thinking behind that decision—-you’re going to spend all your money buying a new piggy bank.  A vicious, irrational cycle.