What word in the preamble of the constitution indicate that the farmers wanted the constitution to last forever?

In the preamble of the constitution the word "posterity" is used to signify that this document will last for generations to come. By using this term, the Framers wanted to give a sense of perpetuity to the constitution as milestone in the configuration of  the United States of America.

Posterity

The word is used in the Constitution to mean that it was also written for future generation. The founding fathers were visionaries that envisaged a document that would create a great republic and hoped that years later people will appreciate it