What is the origin of the phrase "a penny for your thoughts"?

Solution 1:

Googling further, I found this quote from "The Dictionary of Clichés" by James Rogers:

penny for your thoughts — "What's on your mind? (Usually said to someone who is looking pensive.) The saying is from a time when the British penny was worth a significant sum. In 1522, Sir Thomas More wrote (in 'Four Last Things'): 'It often happeth, that the very face sheweth the mind walking a pilgrimage, in such wise that other folk sodainly say to them a peny for your thought.'"

Sadly, Google Books doesn't seem to have scanned that book just yet, but they do have a copy of The Proverbs and Epigrams of John Heywood from 1562 that has this cite:

Fréend (quoth the good man) a peny for your thought.

Solution 2:

The phrase means "Tell me what you are thinking.", and the implication is that you're willing to pay money to know what they are actually thinking.

Looks like the phrase is at least 400 years old, and so getting a precise etymology may be hard.

In 1522, Sir Thomas More wrote (in 'Four Last Things'): 'It often happeth, that the very face sheweth the mind walking a pilgrimage, in such wise that.other folk sodainly say to them a peny for your thought.'

http://www.answerbag.com/q_view/285556