A penny for your thoughts is a quaint way of prompting someone lost in thought to share what’s playing on their mind. Sir Thomas More is attributed with the phrase’s initial usage in 1522. In his book “Four Last Things,” he wrote a variation of the idiom, “A penny for your thought,” where he used “thought” in the singular form rather than the plural “thoughts.” The historical context is that a penny was once a significant amount, suggesting that thoughts were highly valued. The idiom is still regularly used in modern conversation to prompt sharing of ideas or opinions.

Source: Grammarist