Used as an allegory, Mr. Market is an imaginary investor devised by Benjamin Graham and introduced in his 1949 book, The Intelligent Investor.1
The Personal Finance Society, via Internet Archive Wayback Machine. “Mr. Market Turns 70.”
In the book, Mr. Market is a hypothetical investor who is driven by panic, euphoria, and apathy (on any given day), and approaches his investing as a reaction to his mood, rather than through fundamental (or technical) analysis.
Read more at the source: https://www.investopedia.com/terms/m/mr-market.asp