It is also expensive: An online version costs $379 for individual yearly access. Many universities have subscriptions, but to improve access, this year the T.L.L. posted PDFs of entries through the letter P for free online.

The T.L.L. has survived a chaotic century: A significant portion of its staff died in combat at the beginning of the First World War. During the Second, the slips were moved to a monastery to escape the bombing of Munich. In response to postwar nuclear fears, they were copied onto microfilm, which was placed in a bunker below the Black Forest, where it remains, alongside other culturally significant work.

Originally a German state undertaking, the project became international after the Second World War. Its 1.25 million euro annual budget still mostly comes from German taxpayers, but international partners, including the United States, send researchers to Munich.

Judging by the accuracy of previous estimates, the 2050 end date may be optimistic. Many of the researchers at the dictionary say they don’t expect to live to see it finished.

But Christian Flow, a visiting assistant professor at Mississippi State University who wrote a dissertation about the T.L.L., said that its duration is also its strength. “The irony is that the timelessness of the thesaurus,” he said, lay “in its inability to finish itself.”