penny for your thoughts, a - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
What are you thinking about? For example, You've been awfully quiet—a penny for your thoughts. This expression dates from the 1500s and was in John Heywood's 1546 collection of proverbs.