The saying "a penny for your thoughts" is an English idiom simply asking people to volunteer their opinions on an issue being discussed. Though no payment actually changes hands, the phrase has become a regular part of the English vernacular. In modern usage, it is often stated as an indirect way of asking what someone is thinking about or what is bothering them. Its origins are fairly unknown though it dates back to at least the end of the Middle Ages.
Meaning
This phrase is basically a proposal, and the speaker is offering to pay to hear the listener's thoughts. It is an idiom, of course, and not meant literally so no real payment generally takes place. The idea, however, is that the person who says "a penny for your thoughts," wants to know what the listener is thinking about and is showing interest through a symbolic offer of payment. It is also commonly used when someone seems to be deep in thought or troubled by an idea, as a polite way of giving the person an opportunity to express his or her ideas or concerns.
Value
When the saying originated, a penny was worth a lot more than it is in the 21st Century. Therefore, a "penny for your thoughts" likely indicated the thoughts were more valuable to those imploring the listener to give them than they are by today’s standards. The phrase is usually meant as a symbolic gesture and the actual value should typically not be considered. This loss of value can be used derisively, however, often implied through tone of voice; someone can use this phrases in a sarcastic way to indicate that someone's idea is bad or worth a penny in modern value.
Earliest Recorded Usage
A "penny for your thoughts" is a phrase generally credited to a man by the name of John Heywood, who was born sometime just before the 16th century. During his life, he was a writer who penned many plays and a book in 1546, known as A dialogue conteinying the nomber in effect of all the proverbes in the Englishe tongue. As English spelling has changed over the years, later publications were frequently shortened to The Proverbs of John Heywood.
It is likely Heywood did not actually come up with the phrase "a penny for your thoughts." Rather, he was simply the first person, or the earliest person found so far, to have chronicled the phrase in written form. The actual origins of the term are unknown, and since his book was simply a collection of common proverbs and expressions, it was probably familiar to people in the mid 1500s.
Similar Expressions
Another phrase similar to "penny for your thoughts" is offering "your two cents" after making a statement. Someone might give his or her opinion and then say, "that's my two cents," to indicate the value of his or her idea. While, much like a penny, "two cents" is relatively low in value now, it would have been more valuable at one time and the expression is used in much the same way.