Muddy Pen is a term used to describe a situation in which a writer has difficulty expressing their thoughts clearly. The term is derived from the phrase 'muddy pen, ' which was used by writers in the 19th century to describe a situation in which they were unable to write clearly. The crossword solution for Muddy Pen is STY, which stands for 'STYLE, ' and refers to the difficulty of finding the right words to express one's thoughts.