He wrote a dictionary

The Deutsches Wörterbuch, a massive dictionary of the German language by Grimm, was first published in 1854 after beginning in 1838. Jacob Grimm and his brother expected it to span six to seven volumes over a period of ten years. They couldn't finish it since it was started on a scale that was too great for them to handle. As far as Grimm himself was involved, the dictionary has been described as a high-quality compilation of disjointed antique writings.

The dictionary wasn't finished until more than 100 years after Grimm's death, carried on by generations of successors. Jacob Grimm only lived to see the letter F completed. Later researchers finally completed it in 1961 and added to it in 1971. With 33 volumes and about 330,000 headwords, it is still a widely used work of reference today.

Many Germanists carried on the enormous dictionary effort after the death of the two brothers. But it wouldn't be finished for another 107 years after the first volumes were released. The Prussian Academy of Sciences oversaw the creation of the dictionary at the beginning of the 19th century, and renowned members like the physicist Max Planck pushed for its completion. Both East and West Germany carried on with business during the Cold War. On January 4, 1961, the 32nd and last volume were released. Zyypressenzweig was the final submission for Z. (cypress branch).