Q From Brendan Hale: A colleague here in Taiwan has just asked, “What does a penny for your thoughts mean?” I would generally use it to ask someone to tell me what they’re thinking, and my online search tells me that’s the common use. But, so far, I’ve failed to find out the origin of the phrase.
A It’s an odd little idiom, a colloquial way to speak to somebody who’s lost in thought or daydreaming. It is used to ask what they’re thinking about, but as often a gentle way to point out that they’re preoccupied.
This was a long thought to think, and George looked very serious while she was thinking it. Julian looked up and caught her blue eyes fixed on him. He smiled. “Penny for your thoughts!” he said. “They’re not worth a penny,” said George, going red.
Five On A Treasure Island, by Enid Blyton, 1942.
Some people wonder if it might be insulting, since a penny is such a small amount of money, and might produce the sharp response “Is that all you think my thoughts are worth?” That certainly wasn’t the idea behind it, since a penny was worth rather a lot when the phrase was first written down about 1535. It was then a silver coin and experts estimate on the basis of average earnings that it was worth in the region of 1600 modern pence (if the value is estimated on the basis of purchasing power, the figure drops to between 65 and 120 pence). Alas, the idiom hasn’t kept pace with inflation.
We have no idea who invented it. We know it from the works of Sir Thomas More, lord chancellor, humanist, and martyr, which were published posthumously in 1557. He wrote around 1535 that it was used with a note of reproach about a vagrant mind. A little later, it appears in a famous collection:
Wherewith in a great musing he was brought,
Friend (quoth the good man) a penny for your thought.
A Dialogue Containing the Number in Effect of all the Proverbs in the English Tongue, by John Heywood, 1546.