Aha! I knew I just read this somewhere. I've been reading back issues of Take Our Word For It, where they did an article about coin names:

http://www.takeourword.com/TOW132/page1.html

And here's the relevant bit (although I suggest you read the article; it's pretty good):

This name goes back to Anglo-Saxon times when the Old English word for "money" was pening.

Ta-da!